feat/enterpriseportal: define SubscriptionLicenseChecksService (#64396)

This PR defines an Enterprise Portal version of the "license check" API
that is currently bundled in Souregraph as a separate gRPC service that
will be implemented in Enterprise Portal (in a PR I will stack on top of
this one). The API schema is pretty much the same, except:

- removed the `error` field, since I'm pretty sure we can return a real
error instead in a connectRPC implementation for the same effect.
- made `license_key` a parameter, as we already propagate the license
key all over the place (pings, telemetry) - this is a fairly
low-frequency check so there shouldn't be any bandwidth concerns or the
like. For now, this parameter will support the existing format so we can
forward checks in dotcom to EP, but I've set a target removal release

Existing API:
https://sourcegraph.com/github.com/sourcegraph/sourcegraph/-/blob/cmd/frontend/internal/dotcom/productsubscription/license_check_handler.go?L133

Part of https://linear.app/sourcegraph/issue/CORE-227

## Test plan

CI

Author: bobheadxi

lib/enterpriseportal/README.md

@@ -9,7 +9,8 @@ To regenerate all relevant bindings:
 ```sh
 sg gen buf \
   lib/enterpriseportal/subscriptions/v1/buf.gen.yaml \
-  lib/enterpriseportal/codyaccess/v1/buf.gen.yaml
+  lib/enterpriseportal/codyaccess/v1/buf.gen.yaml \
+  lib/enterpriseportal/subscriptionlicensechecks/v1/buf.gen.yaml
 ```
 
 > [!CAUTION]

lib/enterpriseportal/subscriptionlicensechecks/v1/BUILD.bazel

@@ -0,0 +1,45 @@
+load("@io_bazel_rules_go//go:def.bzl", "go_library")
+load("@rules_proto//proto:defs.bzl", "proto_library")
+load("@rules_proto_grpc//doc:defs.bzl", "doc_template_compile")
+load("@rules_buf//buf:defs.bzl", "buf_lint_test")
+
+# Bazel-generated files are different from what is generated locally by buf and
+# causes compilation errors - the next line disables Gazelle-generated Bazel
+# configuration that overrides the generated code that gets committed.
+# https://github.com/sourcegraph/devx-support/issues/932#issuecomment-2103608521
+# gazelle:proto disable_global
+
+proto_library(
+    name = "v1_proto",
+    srcs = ["subscriptionlicensechecks.proto"],
+    strip_import_prefix = "/lib",  # keep
+    visibility = ["//visibility:public"],
+    deps = [
+        "@com_google_protobuf//:field_mask_proto",
+        "@com_google_protobuf//:timestamp_proto",
+    ],
+)
+
+go_library(
+    name = "subscriptionlicensechecks",
+    srcs = [
+        "subscriptionlicensechecks.pb.go",
+        "subscriptionlicensechecks_grpc.pb.go",
+    ],
+    importpath = "github.com/sourcegraph/sourcegraph/lib/enterpriseportal/subscriptionlicensechecks/v1",
+    visibility = ["//visibility:public"],
+    deps = [
+        "@org_golang_google_grpc//:grpc",
+        "@org_golang_google_grpc//codes",
+        "@org_golang_google_grpc//status",
+        "@org_golang_google_protobuf//reflect/protoreflect",
+        "@org_golang_google_protobuf//runtime/protoimpl",
+    ],
+)
+
+buf_lint_test(
+    name = "v1_proto_lint",
+    timeout = "short",
+    config = "//internal:buf.yaml",
+    targets = [":v1_proto"],
+)

lib/enterpriseportal/subscriptionlicensechecks/v1/buf.gen.yaml

@@ -0,0 +1,17 @@
+# Configuration file for https://buf.build/, which we use for Protobuf code generation.
+version: v1
+plugins:
+  - plugin: buf.build/grpc/go:v1.3.0
+    out: .
+    opt:
+      - paths=source_relative
+  - plugin: buf.build/protocolbuffers/go:v1.33.0
+    out: .
+    opt:
+      - paths=source_relative
+
+  # Generate connectrpc bindings in Go
+  - plugin: buf.build/connectrpc/go:v1.16.1
+    out: .
+    opt:
+      - paths=source_relative

lib/enterpriseportal/subscriptionlicensechecks/v1/subscriptionlicensechecks.pb.go

@@ -0,0 +1,35 @@
+syntax = "proto3";
+
+package enterpriseportal.subscriptionlicensechecks.v1;
+
+option go_package = "github.com/sourcegraph/sourcegraph/lib/enterpriseportal/subscriptionlicensechecks/v1";
+
+// SubscriptionLicenseCheckService is the *public-facing* API specification for
+// Enterprise Portal's online license checks.
+//
+// Product implementations can use these endpoints to verify the status of a
+// license.
+service SubscriptionLicenseChecksService {
+  // CheckLicenseKey returns the status of a license key.
+  rpc CheckLicenseKey(CheckLicenseKeyRequest) returns (CheckLicenseKeyResponse) {}
+}
+
+message CheckLicenseKeyRequest {
+  // The signed license key to validate. For backwards compatibility, this
+  // currently also accepts a token of the format 'slk_$hex($sha256(licenseKey))',
+  // but this is deprecated and will be removed in Sourcegraph 5.8 onwards.
+  //
+  // Required.
+  string license_key = 1;
+  // Self-reported Sourcegraph instance identifier (also known as 'site ID').
+  //
+  // Required.
+  string instance_id = 2;
+}
+
+message CheckLicenseKeyResponse {
+  // Valid indicates whether the license key is valid.
+  bool valid = 1;
+  // Reason can contain a human-readable explanation of the response.
+  string reason = 2;
+}