Store `freshli_agent.proto` in this repository

[mscottford]

There is currently no authoritative location for the gRPC service definition that is used by the CLI and the language agents. The file should be moved to this location, and each repository that needs a copy of the file should retrieve it as a build step.

• Remove the file from the Freshli CLI
• Remove the file from the Freshli Java Agent
• Remove the file from the Freshli Dotnet Agent
• Remove the file from the Freshli Python Agent