Add an option to generate enums as pascal case

We're following the style guide outlined in the typescript book https://basarat.gitbook.io/typescript/styleguide#enum

We want our enums to be pascal case but I do not see an option in the apollo cli to generate our enums in that format. I would like to add an option to the `client:codegen` command to adjust enums to be pascal case rather than shout-case.

Start at the client:codegen command entry point and trace how enums are generated. Done means the command accepts an option that generates Pascal Case enums instead of shout-case, while the existing behavior remains available; check the generated output for both modes.

Written by the indexing model from the issue text.