Introduce gRPC Proto Definition

Native support for gRPC in Hive Metastore will give users access to many features including built-in streaming RPC support, default use of HTTP/2 protocol, fine grained security, and more.

This PR will introduce the complete proto file and necessary modifications to pom.xml files to enable gRPC support. It will be followed with PRs that will contain subsets of the Hive Metastore methods along with Thrift to gRPC mappers and unit tests.