AIP-203: require field_behavior

[toumorokoshi]

AIP-203 states:

APIs should use the google.api.field_behavior annotation to describe well-understood field behavior, such as a field's being required, immutable, or output only:

This is not verified by the api-linter today.

[toumorokoshi]

likely we could force this by requiring that at least one of required, optional, output_only, or input_only is set.

[toumorokoshi]

side note for discussion: I'm not sure why optional and input_only is not a valid combination, but TBInvestigated.

[shwoodard]

Would this be ubiquitous or Cloud only?

[toumorokoshi]

I'd say in general these should be ubiquitous unless there's a good reason to scope it.

[shwoodard]

Are there other values that might be reasonable for the set? iirc there was additional values that tf uses.

[toumorokoshi]

fixed with #1149