Use repo assignable / mentionable users in finder

[wd60622]

Describe what this PR does / why we need it

Use repo's assignable / mentionable users to populate finder instead of general GitHub search.

TODO:

• Add a configuration option so that the previous behavior can be used if desired
• Implement for the other pickers / refactor user list logic to separate file. Thoughts on this @pwntester?
• Update any documentation

Does this pull request fix one issue?

Closes #423

Describe how you did it

Pull out the finder creation logic

Describe how to verify it

Use Octo assignee add or Octo reviewer add to see the new behavior.

Special notes for reviews

[pwntester]

It would be great to move the shared code to a shared location so other pickers can reuse it. But since there is no other implementation for fzf-lua yet, Im ok if you want to merge this and let the implementator for fzf-lua to refactor that code.

[pwntester]

@milogert just a heads up about a new implementation of the picker for choosing users in case you can port it to fzf-lua

[wd60622]

Might be good to add to the readme before merge if you think this config change is reasonable @pwntester

[pwntester]

Yep, please update the docs so users know the three options and the differences between them

[wd60622]

I've added to the README. Let me know what you think about the location @pwntester

[pwntester]

I've added to the README. Let me know what you think about the location @pwntester

That should be enough, thanks!

[GrimalDev]

Tested the integration on my config. You guys rule! It is a banger and the documentation is crystal clear.

[pwntester]

Thanks @wd60622!

[milogert]

@milogert just a heads up about a new implementation of the picker for choosing users in case you can port it to fzf-lua

Thanks! I can start taking a look at this 👌