Add support for `submitpackage`

[tcharding]

This was started already on the old repo (rust-bitcoin/rust-bitcoind-json-rpc#23) but I went and move the repo before it got merged.

Just need to port the PR to this repo for further review.

[tcharding]

cc @tnull - the other repo got archived so I can't respond to your question (rust-bitcoin/rust-bitcoind-json-rpc#23 (comment)).

[tnull]

Okay. Feel free to port over the PR, or I can do it, whatever you prefer.