Feature Request: move all .repo-metadata.json into one .repo-metadata-full.json

[bshaffer]

Rather than distribute hundreds of .repo-metadata.json in each repo, configure a single manifest-style .repo-metadata-full.json file (like go does here). This will have the following benefits:

• Much fewer files (remove 150+ .repo-metadata.json files)
• Don't distribute config files in prod packages (right now we have to, for internal/scalability reasons. See build: remove .repo-metadata.json from export-ignore gitattributes config #7276)
• It's simpler to have all this metadata defined in one place