Migrate jProtoc and Canteen to grpc-ecosystem

[rmichela]

With the loss of a consistent release sponsor at Salesforce, it's time to move this repo to the grpc-ecosystem org.

Tasks to do:

• [x] Your project is currently hosted in a public repository.
• [x] Your project's license is the Apache license 2.0.
• [x] Your project doesn't depend on third party libraries with copyleft licenses such as AGPL and GPL.
• [ ] Your project has a top-level README.md with the following information:
  • [x] Project overview
  • [x] How gRPC is used
  • [ ] A list of external project dependencies such as third-party libraries
  • [ ] Build instructions
  • [ ] Automated tests and test instructions
• [ ] Ensure that all project contributors have signed Google's Contributor License Agreement, accepting that all license and patent rights are determined by the CLA under the Apache license.
• [ ] Get written signoff from Salesforce
• [ ] Once you've addressed all of the requirements listed above, complete and submit a gRPC Ecosystem Project Addition Request Form.
• [ ] Update Maven coordinates
• [ ] Update Java package names
• [ ] Add an explicit module-info.java for Java >9 users
• [ ] Secure access to new Maven repository at Sonatype
• [ ] Release version 2.x