## Description

Closes https://github.com/stacks-sbtc/sbtc/issues/1731.

## Changes

1. The signer now ignores the aggregate key field in the sign request
from the coordinator. Instead the signer figures out the aggregate key
themselves.
2. Deprecate the `aggregate_key` field of the
`StacksTransactionSignRequest` struct. The signers will continue to send
the aggregate key in that field but they will not use it themselves.

## Testing Information

This PR adds a new test that the `TxSignerEventLoop` ignores the
`aggregate_key` field.

## Checklist:

- [x] I have performed a self-review of my code