Support message-only compose alias

[caydyan]

Supersedes #3370 with the same fix rebased onto the current develop branch.

Fixes #2185.

This adds message_only as a public compose option alias for return_only_data, matching the issue discussion's request for a compose call that returns just the packed Counterparty message hex.

Details:

• Adds message_only to CONSTRUCT_PARAMS.
• Normalizes message_only=True to return_only_data=True during construct parameter preparation.
• Keeps an explicit return_only_data=True intact even when message_only=False.
• Extends composer unit coverage for both parameter normalization and transaction composition output equivalence.

Tests:

• python -m pytest counterpartycore/test/units/api/composer_test.py -q
• python -m ruff check counterpartycore/lib/api/composer.py counterpartycore/test/units/api/composer_test.py
• python -m ruff format --check counterpartycore/lib/api/composer.py counterpartycore/test/units/api/composer_test.py
• git diff --check

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