Protobuf API V2: Add map, repeated, type conversions

[seena-stripe]

Summary

This PR adds a map and list implementation on top of the go protobuf v2 api, and adds type conversions (protoreflect.Value to/from starlark.Value). This is largely boilerplate, and is unused so it should not have any functional impact on Skycfg today.

• protomodule_map implementation to wrap an underlying starlark.Dict with typechecking on writes. This lines up with protoMap from the existing implementation https://github.com/stripe/skycfg/blob/trunk/internal/go/skycfg/proto_message.go#L869
• protomodule_list implementation to wrap an underlying starlark.List with typechecking on writes. This lines up with protoRepeated from the existing implementation https://github.com/stripe/skycfg/blob/trunk/internal/go/skycfg/proto_message.go#L679
• type_conversions: this implements valueFromStarlark and valueToStarlark like the existing implementation (throughout https://github.com/stripe/skycfg/blob/trunk/internal/go/skycfg/proto_message.go)

Motivation

I am continuing the migration to the go protubuf v2 api, which is partially started on trunk

Tests

• Added tests: I have added tests for the map and list types. type_conversions will be tested implicitly once the message type is added (it's awkward to access right now). These tests cover TestListMutation and TestMapMutation from the existing implementation, but line up with the new protomodule_test functions for messageType, enum and package
• Ran bazel + go build
• I have another branch (https://github.com/stripe/skycfg/tree/seena/protobuf-api-v2) with the complete migration which passes all existing tests under internal/
• I have also done some snapshot tests on our internal usage of skycfg to verify the before/after behavior of the migration is consistent.