feat: Add gravity leaders endpoint

[reidhoch]

Added the new Gravity Leaders endpoint. The NBA defines Player Gravity as "Gravity quantifies how much a player pulls defenders towards them above expected, essentially measuring how much attention they draw compared to what the spacing on the floor predicts."

[reidhoch]

@swar and/or @rsforbes would either of you mind taking a look at this PR and giving feedback? Thanks!

[swar]

I would prefer this be split out into multiple PRs. I see three different things that is trying to be solved in this MR:

1. Adding the new endpoint
2. Updating the poetry lock file with new packages
3. Adding the codeowners file.

I prefer these being targetted towards one task instead of trying to accomplish all of these things in one.

[reidhoch]

Fair enough, feel free to close this PR and I'll open individual ones at a later time.