feat(gax): implement REST URI percent-encoding and dot segment validation

[cy-yun]

Description

This PR implements AIP-136 requirements for custom HTTP bindings, specifically for REST URI percent-encoding and dot segment validation, while preserving backward compatibility for GAPIC client resource names.

Key Changes

1. REST URI Percent-Encoding

   • Values bound to URI variables (* and **) are now correctly URL-encoded.
   • For double wildcard segments (**), forward slashes (/) are preserved, while the individual components between the slashes are properly percent-encoded.
   • This ensures that reserved characters in URIs are safely encoded.

2. Dot Segment Validation

   • HTTP binding values mapped to variables must not contain segments that are exactly . or ...
   • If a variable bound to * or ** contains an exact . or .. segment, the template renderer will now throw an InvalidArgumentException.

3. Backward Compatibility & Resource Name Fixes

   • The original implementation of AIP-136 inadvertently applied URL encoding to resource names used internally and in gRPC clients (e.g., encoding (default) to %28default%29), leading to widespread unit test failures across other GAPIC clients like Firestore and Datastore.
   • This PR resolves those issues by introducing an optional $urlEncode parameter to render() in RelativeResourceTemplate, AbsoluteResourceTemplate, and PathTemplate, which defaults to false.
   • RequestBuilder::tryRenderPathTemplate() has been updated to explicitly pass $urlEncode = true, ensuring that the percent-encoding behavior is safely scoped only to the final REST URI request path generation.

Related Issues

• Implements AIP-136 Custom HTTP Bindings.