Externalize language-agnostic *.proto in a new repo to be single-source-of-truth

[tinker-michaelj]

Summary
The only officially published protobuf assets are in the hapi-proto/ directory of the hashgraph/hedera-services repo and the com.hedera.hashgraph:hedera-protobuf-java-api:* artifacts deployed to Maven Central.

However, the correctness of these *.proto files depend on Java-specific protobuf options, such as,

option java_package = "com.hederahashgraph.api.proto.java";
option java_multiple_files = true;

Therefore, for example, a Go developer cannot use these *.proto files as-is. (Other options may need to be added.)

Possible resolution

• Repurpose the hashgraph/hedera-protobuf GitHub repository to be a source of *.proto files that work for all supported languages.
  • However, do not include any build system-related assets in this repo.
• Adapt the Hedera Services codebase as needed to work with these protobufs.

[tinker-michaelj]

Effective duplicate of #883.