Fix autoboxing of wrapperspb types

[seena-stripe]

Summary

Autoboxing of wrapperspb types was only partially working, allowing msg.f_StringValue = "something" to typecheck (typechecking was done by valueFromStarlark which does the autoboxing) but the assignment was actually assigning "something" a string into the internal fields map. Then when access happens, msg.f_StringValue.value fails because msg.f_StringValue == "something" and does not have .value

This PR removes maybeConvertToWrapper from valueFromStarlark so typechecking fails if the conversion has not been explicitly done before assignment. The conversion is moved earlier into protoMessage.SetField where the list/dict conversions are done.

As an extra fun, there was existing support for converting ["s1", "s2"] into [StringValue{value: s1}, StringValue{value: s2}] which breaks the list assignment semantics. In this case the assignment mutates like map do for None, though it could alternatively copy if we want.

Longer term, I'd really like to get rid of this autoboxing behavior, its convenient but adds a lot of complexity

Tests

I added tests to protomodule_message_test

[kathleen-stripe]

LGTM except for possibly one comment - feel free to send back to me for re-review if no changes are needed