feat: Added GitHub app authentication

[galbirk]

In this PR I added the feature of authenticating to GitHub API with github app and allowing to fetch metrics about it.

Tests:

$ make test                                                                                                                     
?       github.com/infinityworks/github-exporter        [no test files]
?       github.com/infinityworks/github-exporter/config [no test files]
?       github.com/infinityworks/github-exporter/exporter       [no test files]
?       github.com/infinityworks/github-exporter/http   [no test files]
=== RUN   TestHomepage
Created sequence diagram (2641264843_2166136261.html): /Users/galbirkman/projects/github-exporter/test/.sequence/2641264843_2166136261.html
--- PASS: TestHomepage (0.01s)
=== RUN   TestGithubExporter
time="2023-05-21T16:30:35+03:00" level=info msg="Fetching https://api.github.com/repos/myOrg/myRepo?per_page=100 \n"
time="2023-05-21T16:30:35+03:00" level=info msg="Fetching https://api.github.com/repos/myOrg/myRepo/releases \n"
time="2023-05-21T16:30:35+03:00" level=info msg="Fetching https://api.github.com/repos/myOrg/myRepo/pulls \n"
time="2023-05-21T16:30:35+03:00" level=info msg="API data fetched for repository: https://api.github.com/repos/myOrg/myRepo?per_page=100"
time="2023-05-21T16:30:35+03:00" level=info msg="All Metrics successfully collected"
Created sequence diagram (31181356_2166136261.html): /Users/galbirkman/projects/github-exporter/test/.sequence/31181356_2166136261.html
--- PASS: TestGithubExporter (0.07s)
PASS
ok      github.com/infinityworks/github-exporter/test   1.388s

Closes #95

[ben-z]

@henrymcconville can we merge this PR? Would love to have this feature.

[henrymcconville]

Thanks for the PR! I've marked one of my comments IMPORTANT and added a few minor nitpicks that would be good to address but aren't critical. Would you also be able to rebase off the master branch to resolve the conflicts in config.go and go.sum. Thanks!

[galbirk]

Thanks @henrymcconville !
Will take a look and update. Hopefully in the next few days.

[galbirk]

@henrymcconville I removed duplicated code and added some fields to the Config struct.
I was thinking to create GitHubApp struct specifically for GitHubApp configuration to not overload the Config one. WDYT? For now, all added to the Config struct.

[henrymcconville]

@galbirk Think a separate struct would be nicer but happy with it as is for now. Feel free to raise a follow up PR if you're passionate enough about it being separated 😄