Make plugin registries work

BUILD

Makefile

binding.gyp

build.yaml

config.m4

gRPC.podspec

grpc.gemspec

package.json

package.xml

src/core/ext/lb_policy/pick_first/pick_first.c

src/core/ext/lb_policy/pick_first/pick_first.h

src/core/ext/lb_policy/round_robin/round_robin.c

src/core/ext/lb_policy/round_robin/round_robin.h

src/core/lib/surface/init.c

src/core/plugin_registry/grpc_plugin_registry.c

src/core/plugin_registry/grpc_unsecure_plugin_registry.c

src/python/grpcio/grpc_core_dependencies.py

templates/src/core/plugin_registry.template

test/core/client_config/lb_policies_test.c

tools/buildgen/plugins/expand_filegroups.py