WIP: Add GiteaSource

[KennyTK]

Add basic support for Gitea.

TODO:

• [ ] Add support for LDAP based authentication to handle token in memory.
• [ ] Utilize more Gitea features to improve update process.
• [ ] Validate inputs further in constructor and nullability in tasks.
• [ ] Add GiteaSource.md
• [ ] Validate token length.
• [ ] Handle 401 response error in HttpClientFileDownloader
• [ ] Add GiteaRepository
• [ ] Add GiteaCommand
• [ ] Add GiteaCommandTests

[caesay]

Any plans to finish working on this PR?

[KennyTK]

I'll take a hack at everything except LDAP authentication and release notes this weekend.

[caesay]

Closing here. If you'd like this to be merged, please send a PR to the Velopack repo!