AIP-203- Add support for one_of in field behavior

[vishal-chdhry]

Currently, when field behavior REQUIRED is added to every value in a one_of, the implementation in this repository blocks on all values. This issue talks about an alternative approach to solve this: aip-dev/google.aip.dev#1147

But this is not supported by the implementation yet, and therefore there is no way to effectively use REQUIRED in one_of with this implementation.

[github-actions]

This issue has been open for 365 days with no activity. Marking as stale.