Support PROTO related syntax

[apstndb]

https://cloud.google.com/spanner/docs/reference/standard-sql/protocol-buffers

• [CREATE | DROP | ALTER] PROTO BUNDLE statements Support DDLs for Protocol Buffers #176
• CAST AS PROTO Support proto/enum as typename #121
• [NEW operator] Introduce TypelessStructLiteralArg and NewConstructorArg interfaces #183 Support protobuf NEW operators #119
• SELECT AS typename Support SELECT AS VALUE and SELECT AS typename #122
• PROTOs as type name Support proto/enum as typename #121

FYI

These functionalities are under developed also in spansql.

• spanner/spansql: support for Protobuf column-types googleapis/google-cloud-go#10944
• feat(spanner/spansql): add support for protobuf column types & Proto bundles googleapis/google-cloud-go#10945

[apstndb]

Protocol Buffers support seems to be completed!