Add GetObjectDirectorySize RPC

This RPC will be used to determine if a push will
exceed the size limit of a repository.
5 jobs for pb-get-objects-directory-size in 1 minute and 58 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #217899793
build

00:01:11

passed #217899794
check-grpc-proto-clients

00:01:09

passed #219729420
danger-review

00:00:46

passed #217899792
test

00:01:07

passed #217899795
danger-review

00:00:22