Gitea / Gogs plugin

This is the sequel of [the Sentry plugin issue 145](https://github.com/getsentry/sentry-plugins/issues/145)

## Summary

Add a plugin, that is similar to the [GitLab](https://docs.sentry.io/workflow/integrations/global-integrations/#gitlab)/[GitHub](https://docs.sentry.io/server/integrations/github/)/[BitBucket](https://docs.sentry.io/workflow/integrations/global-integrations/#bitbucket) integration, for [Gitea](https://gitea.io/)/[Gogs](https://gogs.io/).

## Motivation

[Gitea](https://gitea.io/) and [Gogs](https://gogs.io/) are git services like GitLab or Bitbucket. They are popular self-hosted and resource friendly alternatives and you will find various instances in the web.

It is obviously useful to integrate Sentry with Gitea/Gogs for companies, universities and other organisations that are hosting their instances.

Until this is fixed, no Gitea/Gogs user can't fulfil what Sentry is suggesting: [linking to a repo](https://docs.sentry.io/workflow/releases/?platform=browser#setting-up-releases).

## Additional Context

### Gitea vs GoGS API

[**Gitea**](https://github.com/go-gitea/gitea/) is a fork of [**GoGS**](https://github.com/gogs/gogs) and Gitea has nowadays about 3'000 patches on top of GoGS, more maintainers, more functionality and bugs fixed that are still present in GoGS (that's the reason why I switched once). So if you are running in some API bug, you should keep that in mind.

### Testing & developing against an API

Also you can simply test the API integration on [the _official test instance_ at `https://try.gitea.io/`](https://try.gitea.io/). There you can also find [the _Swagger API_ definition](https://try.gitea.io/api/swagger). As you can see, JSON and HTTP are protocol wise the only requirements but [there are also various ready-to-use Swagger libraries for Python](https://swagger.io/tools/open-source/open-source-integrations/#python).

If you rather want to use a _local_ Gitea instance, you might want to use [the official _Docker image_](https://hub.docker.com/r/gitea/gitea/) (there's also a [`docker-compose.yml`](https://hub.docker.com/r/gitea/gitea/#quick-set-up) provided that you can use).

There's also more on [API authorisation in the documentation](https://docs.gitea.io/en-us/api-usage/).

**PS:** People reading this, might also be interested [in this issue (a Sentry webhook for Gitea)](https://github.com/go-gitea/gitea/issues/7548).