Add GSIS IDs to Player Contracts

[armstjc]

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

Within the nflverse datasets, there is no direct way to map nflverse contract information to any other dataset as is, because there is no column in that dataset that holds GSIS ID information.

2. Describe the solution you'd like

A column named [gsis_id] added to the nflverse contract information, to make it easier to map player contract information to other parts of the nflverse project.

3. Describe alternatives you've considered

None.

4. Additional context

Found this problem when trying to create a local nflverse database.