Merge bitcoindevkit/rust-hwi#104: feat: add `HWISigner`, moved from `… #441

Job	Run time
test-readme-examples	5m 35s
fmt	6s
build	2m 46s
test-emulators (stable, miniscript, trezor)	6m 19s
test-emulators (stable, miniscript, ledger)	1m 33s
test-emulators (stable, signer, trezor)	6m 26s
test-emulators (stable, signer, ledger)	1m 42s
test-emulators (1.63.0, miniscript, trezor)	7m 55s
test-emulators (1.63.0, miniscript, ledger)	3m 34s
test-emulators (1.63.0, signer, trezor)	7m 52s
test-emulators (1.63.0, signer, ledger)	3m 48s
	47m 36s

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