Add MethodExtension to ServiceImplInstance

[Quinn-With-Two-Ns]

Add MethodExtension to ServiceImplInstance. This allows users, like the Temporal SDK, to define custom annotations on service handlers.

This will allow us to define Nexus Service like so.

  @ServiceImpl(service = SugarService.class)
  public static class SugarServiceImpl {
    @TemporalOperation
    public TemporalOperationResult<String> start(
        TemporalOperationStartContext ctx, TemporalNexusClient client, String input) {
      return client.startWorkflow(
          SugarTargetWorkflow.class,
          SugarTargetWorkflow::run,
          input,
          WorkflowOptions.newBuilder().setWorkflowId("sugar-" + ctx.getRequestId()).build());
    }
  }

See also temporalio/sdk-java#2928

[bergundy]

Let's mark this as experimental. We should do this for all of the non final APIs like the ServiceImpl.