java_grpc_library.bzl: Allow toolchain to use annotation processors

While not easy to use because java_grpc_library() uses a fixed
toolchain, it is possible for downstream users to apply a patch to the
repo to add their own annotation processors. This feature was added
inside Google so exporting it reduces the diff between internal and
external and causes no harm.

cl/280287611

java_grpc_library.bzl

@@ -3,6 +3,7 @@
 _JavaRpcToolchainInfo = provider(
     fields = [
         "java_toolchain",
+        "java_plugins",
         "plugin",
         "plugin_arg",
         "protoc",
@@ -14,6 +15,7 @@ def _java_rpc_toolchain_impl(ctx):
     return [
         _JavaRpcToolchainInfo(
             java_toolchain = ctx.attr._java_toolchain,
+            java_plugins = ctx.attr.java_plugins,
             plugin = ctx.executable.plugin,
             plugin_arg = ctx.attr.plugin_arg,
             protoc = ctx.executable._protoc,
@@ -39,6 +41,10 @@ java_rpc_toolchain = rule(
             default = Label("@com_google_protobuf//:protoc"),
             executable = True,
         ),
+        "java_plugins": attr.label_list(
+            default = [],
+            providers = [JavaPluginInfo],
+        ),
         "_java_toolchain": attr.label(
             default = Label("@bazel_tools//tools/jdk:current_java_toolchain"),
         ),
@@ -104,6 +110,7 @@ def _java_rpc_library_impl(ctx):
         source_jars = [srcjar],
         output = ctx.outputs.jar,
         output_source_jar = ctx.outputs.srcjar,
+        plugins = [plugin[JavaPluginInfo] for plugin in toolchain.java_plugins],
         deps = [
             java_common.make_non_strict(deps_java_info),
         ] + [dep[JavaInfo] for dep in toolchain.runtime],