feat: initial vetiver_pin_metrics implementation

[machow]

Quick note on pre-commit:

• I think some of the current hadn't been run through pre-commit, so were tweaked a bit by black.
• I set flake8 to allow lines to be 90 characters so it wouldn't error for somethings black won't format (docstrings etc..). We can remove and re-format them by hand if needed!

TODO:

• : If people save metrics as a CSV, then we need to parse their date index as a datetime. (I set everything to use type="arrow" as a placeholder for now, so tests would pass. But we should change before merging).
• : Need to test different overwrite conditions (e.g. when it should error out, check that it works as expected).

[machow]

See discussion in issue above -- we need to make sure we sync up strategy for specifying pin types with vetiver-r!

[isabelizimm]

Commented in issue, but putting over here for record-keeping--I think deciding pin type based on the original pin is the right way to go :D Looks like the R side will have dots to match this

[machow]

@isabelizimm I think this is ready--feel free to add commits + merge, or if there's anything to change I'm happy to do it!

[isabelizimm]

lgtm! arrow handling can come later since I currently do not have it implemented anywhere else in vetiver 🚢

[machow]

Ah yeeahhh! This should handle arrow (there aren't arrow tests here, since it's not a CI dep, but there are arrow tests in pins)