feat: handlers for assign, deassign, extend and withdraw commands

[achyuthcodes30]

Task

Parse the incoming webhook event from an issue comment and perform one of the following:

1. Assign an issue to a contributor iff they are not currently assigned to another issue and the request is from a maintainer
2. Deassign a contributor from an issue if they are currently assigned and the request is from a maintainer
3. Extend time assigned to a contributor for an issue if the request is from a maintainer
4. Withdraw - if contributor assigned to an issue requests to withdraw

Each handler in turn makes calls to logic in #2 and #1

From Linear