Introduce io.grpc.Uri. (#12535)

`io.grpc.Uri` is an implementation of RFC 3986 tailored for grpc-java's
needs. It lifts some of the limitations of `java.net.URI` that currently
prevent us from resolving target URIs like `intent:#Intent;...` See
#12244 for more.

Marked `@Internal` for now but the plan is to eventually use this to
replace `java.net.URI` in our public APIs such as NameResolver.Factory.

Author: jdcormie

api/build.gradle

@@ -73,6 +73,7 @@ tasks.named("javadoc").configure {
     exclude 'io/grpc/Internal?*.java'
     exclude 'io/grpc/MetricRecorder.java'
     exclude 'io/grpc/MetricSink.java'
+    exclude 'io/grpc/Uri.java'
 }
 
 tasks.named("sourcesJar").configure {