Add template-files composite action

[kenodegard]

Description

We've been using github.com/BetaHuhn/repo-file-sync-action to synchronize files from conda/infrastructure for a while now and have found that it doesn't perform in the way we need it to (bulk syncing with templating), the upstream author is not particularly responsive anymore, and since the upstream action is written in NodeJS/Typescript its not particularly simple to fork and fix.

Thus we implement a small composite action that implements a pull process and uses Jinja as the templating engine. Since this is a decentralized pull process this also gives more control to individual repos instead of having a central sync conductor.

[jezdez]

Some nits on naming etc but I think that's a great start!

[kenodegard]

I have verified that the workflow still works in https://github.com/conda-sandbox:

• https://github.com/conda-sandbox/upstream mocks https://github.com/conda/infrastructure
• https://github.com/conda-sandbox/downstream mocks any of the conda projects, e.g.: https://github.com/conda/conda

See a scheduled run (where new updates are fetched from upstream): conda-sandbox/downstream#19
And a manually triggered run (where we enable nested templating features): conda-sandbox/downstream#20