Revert "Revert #19704 and #20060"

bazel/grpc_build_system.bzl

gRPC-ProtoRPC.podspec

gRPC-RxLibrary.podspec

gRPC.podspec

src/compiler/objective_c_generator.cc

src/compiler/objective_c_generator.h

src/compiler/objective_c_plugin.cc

src/objective-c/BUILD

src/objective-c/GRPCClient/GRPCCall+ChannelArg.h

src/objective-c/GRPCClient/GRPCCall+ChannelArg.m

src/objective-c/GRPCClient/GRPCCall+ChannelCredentials.h

src/objective-c/GRPCClient/GRPCCall+ChannelCredentials.m

src/objective-c/GRPCClient/GRPCCall+Cronet.h

src/objective-c/GRPCClient/GRPCCall+Cronet.m

src/objective-c/GRPCClient/GRPCCall+GID.h

src/objective-c/GRPCClient/GRPCCall+OAuth2.h

src/objective-c/GRPCClient/GRPCCall+Tests.h

src/objective-c/GRPCClient/GRPCCall+Tests.m

src/objective-c/GRPCClient/GRPCCall.h

src/objective-c/GRPCClient/GRPCCall.m