[land-bot] Pass committer name/email to land bot endpoint
[land-bot] Pass committer name/email to land bot endpoint

Summary: The land endpoint requires these fields now.

Test Plan:
Used to land D6796:
`arc land-via-bot --revision D6796`

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6799

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