[owners] Support top-level owners file reviewerTeam property

[rcebulko]

Per #499 , there is a need to have the owners check fail if a PR has approval from the OWNERS of the file, but not from someone in ampproject/reviewers-amphtml (or a comparable reviewer whitelist for other repositories). By integrating this into the owners check, we enable the repository to allow write-access to anyone.

The proposed solution is to allow a reviewerTeam property in the repository root OWNERS file, which can specify a GitHub team name. The owners check will report a failure for any PR unless at least one member of this team has reviewed it.

In any non-root owners file, this key will report an error and be ignored. If no such key is present in the repository root owners file, the condition will not be included.

[rcebulko]

🙌