add action to post binder link on each PR

[Eduardo Blancas]

we want to create a github action that posts a link to binder whenever someone opens a PR, this will allow us to easily test things

some notes:

1. since this requires a github token, I believe the trigger should be pull_request_target
2. the host should be binder.ploomber.io
3. we also need to add an environment.yml, similar to this one
4. and a postBuild like this

more information

https://mybinder.readthedocs.io/en/latest/
https://mybinder.org/

some related code:
https://github.com/jupyterlab/maintainer-tools/blob/main/.github/actions/binder-link/action.yml

[Anirudh Iyer]

Working on it !

[Eduardo Blancas]

any updates here? @AnirudhVIyer

[Anirudh Iyer]

@edublancas I had paused the work on this, will resume after my current PR.