AIP-163: lint validate_only optional

[toumorokoshi]

AIP-163 notes that validate_only is optional (in the example and lack of text saying required): https://google.aip.dev/163

We should lint for this (or double-check that this isn't covered by some other AIP around request fields being required.

[toumorokoshi]

Upon further inspection, I think we can enforce this via a more powerful check enforcing the fields that can be required in a request.

Added one for create with #1142. I'll look at creating one for Get, Update, Delete, and List as well.

[Juneezee]

We can close this issue. Already implemented in #1156