fix(Coder plugin): Update integration with Backstage's identity API

[Parkreiner]

Closes #116

Changes made

• Updated all query logic to start using Backstage's Identity API properly
• Added mock implementation of Identity API for tests
• Added "pseudo-middleware" for MSW to ensure that our tests will fail if our main code fails to pass along the bearer token generated by the Identity API

Notes

• The implementation in the main UI code is a little bit clunky. This is the sort of thing that the API refactor is meant to help clean up.
  • Once the refactor is done, bringing in and updating new Backstage APIs should be a lot more straightforward and plug-and-play
  • This PR is mainly meant to be a stopgap until the API refactor is fully completed. I'll make sure that the refactor includes equivalent identity logic

[Parkreiner]

Not a huge fan of needing to bust out the any type for these MSW helper types, but it's what MSW itself is using, and trying to make things more specific got nasty

[Parkreiner]

@code-asher Would definitely appreciate your perspective on the token stuff. I was aware of bearer tokens, but for all the companies I've worked for so far, they were abstracted out enough on the frontend that I never needed to touch them

[code-asher]

Looks good to me!