@mainnet-pat already did work on a PR to add support for placeholder-based artifacts

opening an issue for this work so we have a place to discuss the feature and plan it in in our milestones

I'm making this a child issue of "better support for hand-written/hand-optimized contract bytecode"

The description/rational for the PR is added below

Placeholder-based artifacts are those which might be produced elsewhere or contain raw bitcoin script. Placeholders are used to insert the properly encoded variables at certain places in the script to produce a valid contract.

This pattern first emerged in our work for bch-xmr atomic swaps, now in another project where I had to support the EC-compatible 2 of 3 multisig contract.

Also add a sample 2 of 3 multisig contract and tests to highlight the feature and demonstrate the usage patterns.