Follow up tracking for PR #8754 (Outbound Remote Signer)

In PR #8754, we introduce a new alternative remote signer implementation called an “Outbound Remote Signer.”

This issue is intended to track any feedback from that PR that we’ve decided is better addressed in follow-up PR(s) rather than in #8754.

Links to the relevant feedback are listed below:

• https://github.com/lightningnetwork/lnd/pull/8754#discussion_r3025905174

• https://github.com/lightningnetwork/lnd/pull/8754#discussion_r3017807157 - Specifically the part about "Sharing the main chain's middleware state"

• https://github.com/lightningnetwork/lnd/pull/8754#discussion_r3162586973 - Make sure on the remote signer side that the remote signer refuses to re-sign MuSig2 requests.