allow `optional` proto fields that are annotated as `REQUIRED`

[vchudnov-g]

Unit tests fail when a field is proto3-optional but has (google.api.field_behavior)=REQUIRED. In spite of the fact the names sound like they are referring to the same thing, they're not, in fact. The former tracks field presence, while the latter tracks the service's expectation. See https://google.aip.dev/149#field-behavior-and-optional.

[vchudnov-g]

Note that his also seems to apply to optional REQUIRED fields. I had captured this in my notes but apparently it didn't get fixed here.