Add support for `testmempoolaccept`

[tnull]

In #6 (comment) we briefly discussed whether to add support for testmempoolaccept, concluding that it won't be added to v1.6.

It however now turned out that the Payjoin folks have had a need for this for some time and previously opened PRs to add support to individual implementations (see https://github.com/orgs/payjoin/discussions/900, romanz/electrs#1212, cculianu/Fulcrum#44).

It would be great if it could be added as part of v1.7 / #2.

cc @DanGould @nothingmuch

[SomberNight]

In #6 (comment) we briefly discussed

link does not work due to some bug in github webui. see #6 (review)

[SomberNight]

testmempoolaccept in Bitcoin Core is surprisingly complex.
I played around with it and here are some examples re how it behaves:
https://gist.github.com/SomberNight/ed874d1113725a5acad414386f813928

I tried to strip it down. Both to simplify it and as the protocol should not depend on bitcoind-implementation details when possible.

Here is the spec I propose for it:
e3bc307
(part of #2)

And as code is usually clearer than words, here is the server-side implementation in e-x:
spesmilo/electrumx@3858c82
(part of spesmilo/electrumx#303)

Please review/see if the spec I wrote for it looks good.

[tnull]

Thanks! I think that looks good from my side, but I'll let other comment on whether stripping the vsize and fees fields is acceptable or if that breaks their use case.