Modularize R API wrapper functions

[bmeluch]

Currently, the taxonomy and NOM R notebooks define functions for interacting with the NMDC API. The functions are essentially the same. (e.g. cell 2 here)

Rather than copy pasting the same code into every new notebook, it would be better to pull those functions out into an R package accessible by all notebooks in this repo.

Completion criteria:

• An R package exists in this repo containing the API interaction functions we've written so far
• The existing notebooks use functions from the package and no longer define them internally
• It plays nicely with our renv .lock file (someone newly cloning the repo can run the notebooks after using renv::restore() )