feat(generator): add Go iam service generation

[codyoss]

It compiles, did not test running it.

• Added lightweight wkt package for Go
• Added Codec fn for appending imports
• Added Go codec opts for mapping imports

Updates: #209

[codyoss]

@coryan I will fixup a couple things here and then wait to merge until we have time to talk about your concerns.

[codyoss]

@coryan was able to factor out the controversial bit, will be merging.