chore: add temporary replace directive for SDK CLI auth support

Add replace directive to use SDK fork with CLI authentication support
from PR stackitcloud/stackit-sdk-go#3865 until it's merged and released.

This allows the provider to be built and tested with the CLI auth
functionality before the SDK changes are officially released.

The replace directive references commit 25b6b99bd648 from
github.com/franklouwers/stackit-sdk-go/core which includes the
core/cliauth package and config.WithCLIProviderAuth() function.

Once SDK PR #3865 is merged and a new SDK version is released,
this replace directive should be removed and the provider updated
to require the new SDK version.

Author: franklouwers

go.mod

@@ -102,3 +102,9 @@ require (
 )
 
 tool golang.org/x/tools/cmd/goimports
+
+// Temporary: Use local SDK with CLI auth support until SDK PR #3865 is merged
+// See: https://github.com/stackitcloud/stackit-sdk-go/pull/3865
+// For testing, use: replace github.com/stackitcloud/stackit-sdk-go/core => ../stackit-sdk-go/core
+// For CI/others: replace github.com/stackitcloud/stackit-sdk-go/core => github.com/franklouwers/stackit-sdk-go/core v0.0.0-20251127223915-25b6b99bd648
+replace github.com/stackitcloud/stackit-sdk-go/core => github.com/franklouwers/stackit-sdk-go/core v0.0.0-20251127223915-25b6b99bd648