Better capture information jointly shared by load_players() and load_rosters()

[john-b-edwards]

Noticing a ton of missing birthdates for older players in nflreadr::load_players() that is readily available in nflreadr::load_rosters(). We should probably do some kind of full_join + coalesce to get a more complete picture of biographical information. Leaving this here as a reminder for myself to tackle this at some point soon.