Add GitHub action that refreshes external context

[matentzn]

You can sync the bioregistry context like this:

make sssom/external_context.py -B

We should add a github action that does this and creates a pull request so we can always be informed of changes in the bioregistry context.

[matentzn]

Probably makes sense to now, that we know how this works, simply add the context as a resource rather than hacking it into a variable in some py file