Add optional txfee property for direct-send wallet RPC

[kristapsk]

Resolves #1360. Jam wants it for joinmarket-webui/jam#678.

[kristapsk]

@theborakompanioni Please test. I only checked that this doesn't break wallet RPC tests, which covers only existing scenario with no custom tx fee specified. Two things should be tested: 1) that tx fee is actually changed, 2) that it's not changed permanently in backend and is reverted to config defaults after tx is made.

[theborakompanioni]

@theborakompanioni Please test. I only checked that this doesn't break wallet RPC tests, which covers only existing scenario with no custom tx fee specified. Two things should be tested: 1) that tx fee is actually changed, 2) that it's not changed permanently in backend and is reverted to config defaults after tx is made.

I'll still need some time to provide proper feedback, but my early tests were successful. So nice!
Will let you know as soon as possible. Thanks @kristapsk 🙏

[AdamISZ]

Thanks for the patch @kristapsk , it's not difficult to imagine why it is wanted (given config edits are janky).

[kristapsk]

Addressed review comments.

[AdamISZ]

tACK d8f1fc4

(this is a bit naughty, I did not test that, but the previous commit, but the only difference is a single str() cast to correct the error, so I don't think it's so bad...)