Stardoc protos and ModuleInfoExtractor / starlark_doc_extract should export parameter type annotations for builtin functions

If we want to documenting builtin functions using ModuleInfoExtractor / starlark_doc_extract (e.g. for the BUILD language API pages of the build encyclopedia), we need to support annotations for parameter types - since builtin functions (unlike ordinary Starlark-defined functions) have them.

The proto representation of type annotations we pick needs to be flexible enough for future needs - there is community pressure to add type annotations to ordinary Starlark-defined functions too.

Two reasonable possibilities are

* flat string - this is what src/main/protobuf/builtin.proto does; maximally flexible, but requires the consumer to do parse the string
* tree of nodes - easier to parse for a consumer, but more of a maintenance burden for us (since new types will inevitably appear and will need to be supported)