Add tests to ensure no copy on assignment, fix map

[seena-stripe]

Summary

This PR adds tests to ensure protoMap, protoList and protoMessage are not copying values on assignment and corrects protoMap. Yesterday I learned Skycfg previously did this sometimes, e.g. in the old implementation this would happen:

# v0.1.0 Behavior
msg1.r_string = ["a","b"]
a = msg1.r_string
msg2.r_string = msg1.r_string
a.append("c")

# a = [a, b, c]
# msg1.r_string = [a, b, c]
# msg2.r_string = [a, b]

(

skycfg/internal/go/skycfg/proto_message.go

Line 518 in 677b39f

case *starlark.List:

)

The re-implementation fixed this for lists but not for maps incidentally so I added tests to make this behavior explicit. AFAICT from testing starlark, starlark does not copy on assignment for lists and dictionaries (like python) and seems to be what is described in https://github.com/bazelbuild/starlark/blob/master/spec.md#value-concepts

Tests

Added tests when assigning skycfg and starlark values into a skycfg object

[kathleen-stripe]

LGTM - one non-blocking comment. Thanks for fixing this behavior and adding test coverage!

[com6056]

@seena-stripe you mentioned that the old implementation

skycfg/internal/go/skycfg/proto_message.go

Line 518 in 677b39f

case *starlark.List:

still has this issue, unfortunately we can't upgrade to the new implementation quite yet due to k8s still using gogoproto.

Do you happen to know how to fix this in the old implementation? I've tried a few things and couldn't quite get it working, was curious if you already knew how to solve it though.