Add target for a table of ScienceBase date last modified

[jds485]

There is a way to check the date that a ScienceBase item was last modified with ​item_get_fields(<sb_id>, 'provenance')​.

So, I think we can make a target for a table containing the provenance and other ScienceBase item information, and pass that table into the download target. In the download target, we can re-download only those items whose date changed.

Suggested steps:

• Add a target that adds a provenance column to NHDVarsOfInterest.csv. This column could also be added within the p1_vars_of_interest target, but I think a new target before p1_vars_of_interest would be better because the provenance should always be built when the pipeline is run. Using cue = tar_cue(mode = "always') should work to do that check.
• Modify fetch_nhdv2_attributes_from_sb to check the provenance column before downloading items from ScienceBase

@lekoenig, @msleckman - tagging for your awareness of this option. Does this sound good to you?