feat(spanner/spansql): add support for protobuf column types & Proto bundles

[dfinkel]

feat(spansql): CREATE/ALTER/DROP PROTO BUNDLE

Add support for parsing and serializing CREATE, ALTER and DROP PROTO BUNDLE DDL statements.

feat(spanner/spansql): support for protobuf types

Now that Spanner supports protobuf message and enum-typed columns and casts, add support for parsing those those types.

Since protobuf columns aren't distinguished by a keyword, adjust the parser to see any unquoted identifier that's not a known type as a possible protobuf type and loop, consuming .s and identifiers until it hits a non-ident/. token. (to match the proto namespace components up through the message or enum names)

To track the fully-qualified message/enum type-name add an additional field to the Type struct (tentatively) called ProtoRef so we can recover the message/enum name if canonicalizing everything.

closes: #10944

[rahul2393]

@dfinkel tests failed, please check and fix them

[dfinkel]

Thanks @rahul2393 for the prompt review and approval!

Sorry about the test failures! It looks like an else if block got lost while I was cleaning up the commits before sending it out for review. (then I absentmindedly ran the tests for the wrong directory :face_with_diagonal_mouth: )

The tests for this package now pass locally.

[dfinkel]

Friendly ping @rahul2393

(thanks for the prompt approval)

[dfinkel]

@rahul2393 if you have a few minutes, can you make another pass over this PR?

[dfinkel]

Thanks @rahul2393 !

[apstndb]

I have re-used this tests for another purpose( https://github.com/cloudspannerecosystem/memefish/issues/115 ) , so it is better to contain only valid cases. Would you fix it? @dfinkel If not, I will send PR.

[dfinkel]

Thanks @apstndb, I definitely copied those cases from the documentation. I can put together a PR to switch those to something that's actually valid tomorrow.

(I think I meant to verify that those were valid before sending this PR out but as usual other things intervened and that got lost)

[dfinkel]

I just opened a PR with fixes for some bugs I ran into while using the features in this PR (including fixing the tests that @apstndb pointed out): https://github.com/googleapis/google-cloud-go/pull/11279

The most problematic bug is the fact that NOT NULL protobuf-typed columns don't get parsed correctly because I had accidentally made the parser too greedy.

Thanks in advance!