Add pfr_id to players dataset

[alecglen]

1. Is your feature request related to a problem? Please describe.

   Currently you have to join across e.g. rosters or ff_playerids to get from players to pfr_advstats via IDs.

2. Describe the solution you'd like

   Add a pfr_id column to the dataset returned by load_players() to enable a direct join to PFR data.

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Not a high priority, just a convenience ask if it's not too much effort.

[mrcaseb]

PFR ID already is on the list of IDs we plan to add. Related issue nflverse/nflverse-rosters#72

I am going to close this as it is more or less a duplicate of the above mentioned issue.

[alecglen]

🤦 missed that, thanks @mrcaseb

[mrcaseb]

No worries. It was in another repo so I don't expect you to find it!