Protobuf API V2: compatibility fixes

[seena-stripe]

Summary

Two fixes to maintain compatibility with the old implementation

• Add json module members for compat (852f982):
  json.marshal became json.encode. Added these aliases back like we do for yamlModule
• Allow constructing message maps with None (416b96c):
  Skycfg previously allowed setting proto2 message values with None, effectively unsetting them. For maps, this set them to be nil, so

# skycfg
msg.map_submsg = { 
  "a": None
}

became

&pb.MessageV2{
  map_submsg: map[string]*pb.MessageV2{
    "a": nil,
  }
}

With protoreflect, map.Set only accepts a protoreflect.Value https://pkg.go.dev/google.golang.org/protobuf/reflect/protoreflect#Map.Set (a struct, cannot be nil) so my two options are 1) create an empty &pb.MessageV2{} or treat None as not setting the key. I went with the latter, to make it behave like protoMessage but neither option seems great

• Allow set to None in proto2 mode for scalar values (873051b): this adds back the behavior from

  skycfg/internal/go/skycfg/proto_test.go

  Line 1069 in e785c6f

  func TestUnsetProto2Fields(t *testing.T) {

  and

  skycfg/internal/go/skycfg/proto_test.go

  Line 870 in e785c6f

  // Repeated and map fields can't be assigned `None`. Scalar fields can't be assigned `None`

Tests

I added a test for compatibility

[kathleen-stripe]

thanks for including this comment!