feat(AIP-203): introduce IDENTIFIER field behavior

[toumorokoshi]

Identifier fields are often multi-modal: they are required or optional
in various situations. Adding an explicit field behavior for these
fields helps eliminate ambiguity for an author to fully annotate field
behavior on all request fields, as well as clarify which fields to use
in identifying the resource for clients.

fixes #1176.

[noahdietz]

Can you include the Compatibility guidance in here as well (you documented it in the proposal doc).

Also, can you update other AIPs that portray the google.api.resource with a name field to include the IDENTIFIER value?

[noahdietz]

Can you include the Compatibility guidance in here as well (you documented it in the proposal doc).

Also, can you update other AIPs that portray the google.api.resource with a name field to include the IDENTIFIER value?

Just friendly reminder that these would be nice too :) At least the first one.

[toumorokoshi]

Can you include the Compatibility guidance in here as well (you documented it in the proposal doc).
Also, can you update other AIPs that portray the google.api.resource with a name field to include the IDENTIFIER value?

Just friendly reminder that these would be nice too :) At least the first one.

good calls! done.

On the meta level: I think we have name in multiple places where it doesn't have to be. I started adding a comment to the effect of "here are the standard fields like google.api.resource and name" so we don't have to worry about updating every example. WDYT?

[noahdietz]

I think we have name in multiple places where it doesn't have to be. I started adding a comment to the effect of "here are the standard fields like google.api.resource and name" so we don't have to worry about updating every example. WDYT?

Oh man yeah there are a lot of spots...perhaps you are right this could be annoying to update in the future as well.

[toumorokoshi]

done and done! please take another look.