Add a programmatic API

[remilapeyre]

We are using Atlantis in our pull requests and for a weekly review of unapplied changes our drift of our configuration. To do this, we open empty PRs so that Atlantis plan all our projects and comment with the changes.

This has several drawbacks: we have to create a useless branch and open an empty PR and Atlantis takes a lock on all our projects that only gets released once we close them and it refuses to look for changes if another PR is already opened.

We could simplify this by having a /plan endpoint we could programmatically call once a week to know whether some projects have unapplied modifications. Ideally it would take a list of repositories and whether, plan everything and return the result as a JSON payload.

[remilapeyre]

Ok, we were also needed an /apply endpoint, I've made a POC at https://github.com/runatlantis/atlantis/pull/997/files.

Since Atlantis expect pretty much everywhere to have a pull request, it's not pretty but it does work.

I did not make huge changes to the interfaces and used type hints instead. Now that it's working, I think we get a better idea of what needs to be updated for this to be implemented properly.

[nitrocode]

This seems like it has been implemented by the linked pr #2455.