Create `/pack` method

[pinnate-modo]

Just as we can pass a datahex to /unpack it'd be great to have a method to which we can pass a Counterparty message and return the hex-encoded message.

[loon3]

might make sense to just add a messageonly flag to the existing v2 API compose calls

[caydyan]

Submitted PR #3370 for this: #3370

Bounty payment address (BTC): bc1qev5ant33v5y89qqjvcf4mh9hlax5svqf5xd7gc

[caydyan]

Submitted develop-based replacement PR #3416 for this: #3416

This supersedes the earlier master-based #3370.

Bounty payment address (BTC): bc1qev5ant33v5y89qqjvcf4mh9hlax5svqf5xd7gc