feat: Add `get_mb_access_token` function + add logo/attribution parameters to `static_mapbox`

[elipousson]

I should have worked on more urgent tasks this morning but I wanted to get the logo/attribution feature while the code was still fresh in my mind. I also added a utility function that reduces some duplicative code in accessing the saved environmental variables.

Here are the details from my commit messages:

• refactor: add get_mb_access_token to other functions using access tokens
• fix: rework the secret_required parameter for get_mb_access_token
• feat: Add attribution parameter to addMapboxTiles
• style: adjust spacing/white space

[walkerke]

perfect, thanks @elipousson!

[elipousson]

Awesome. Incidentally, I got started on package development as a direct result of participating in a workshop you led on this package way back in October 2020. So glad to give back to a project that really had a big influence on my own work!