feat: Add github-upload-public-key module

[f0ssel]

Automates uploading Coder public key to Github so users don't have to.

First build:

Subsequent builds:

Github Auth not linked:

Closes https://github.com/coder/customers/issues/542

[f0ssel]

Working on adding some tests

[matifali]

I suggest we generalize GitHub URL too by making it an input to the module or better deducing it from the configured GitHub external authentication.

[kylecarbs]

This is absolutely sick!

[kylecarbs]

Minor language things that I think could be helpful!

[f0ssel]

I realize there is a conflict between making tests pass and the script actually working in workspaces, I think I need to handle the terraform injection (I didn't know this was actually happening at first) as well as env vars for tests.

[f0ssel]

Okay input variable issues are fixed, screenshots are updated, I think this is g2g but I'll wait until the eod to merge so yall can take a look at the last couple changes since review.