backfill .repo-metadata.json files missing support URLs.

[vchudnov-g]

As we were developing and reviewing #12703, we saw that a lot of of packages in this repo (and in the split-repos) don't have enough data in their .repo-metadata.json files to be able to extract the support URLs we want to present to users: listing API issues, creating an API issue, listing client-library issues.

We need to audit those metadata files to fill in the missing information. This could benefit all languages, and could be pursued as a cross-language project.