Fixes#797.
Adds `custom_change_addr` argument to `direct_send()`
joinmarket-qt: Adds input field for optional external change address
joinmarket-qt: Better handle PayJoin/CoinJoin state changes for changeInput widget
Adds `custom_change_address` argument to Taker constructor and use it in joinmarket-qt
Custom change also allowed in sendpayment CLI with `-u` flag (not
supported in tumbler).
Explicitly disallows using custom change with BIP78 Payjoin, though that
could change later.
Both sendpayment and CLI provide detailed warnings to avoid misuse. In
particular, they have an extra warning for using a nonstandard or
non-wallet scriptpubkey type.
Setting custom change to the recipient address is explicitly forbidden.
Tests: Adds custom_change usage test in test_taker.
csH7KmCC9
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