Add ChannelMonitor justice tx API for simplified watchtower integration

Adds sign_initial_justice_txs(), sign_justice_txs_from_update(), and
get_pending_justice_txs() to ChannelMonitor, enabling Persist implementors
to obtain signed justice transactions for both to_local and HTLC outputs
without maintaining external state.

Storage uses cur/prev counterparty commitment fields on FundingScope,
matching the existing pattern and supporting splicing. The API is
crash-safe: commitment data is cloned rather than consumed, and
get_pending_justice_txs() allows recovery after restart.

Simplifies WatchtowerPersister in test_utils by removing manual queue
and signing logic.

Addresses feedback from lightningdevkit/ldk-node#813 and picks up
the intent of #2552.

Author: FreeOnlineUser