Add `joinpsbts` functionality

[St333p]

Bitcoin Core has a function that allows to merge multiple unsigned PSBTs into one which contains the union of inputs and outputs from the input PSBTs. This is relevant for every usecase in which the set of inputs and outputs of a transaction is collaboratively defined by a set of entities that do not necessarily have a coordinator; for instance it is often used in coinjoin/payjoin transactions.

This PR aims to bring a similar functionality to btclib, choosing flexibility where possible but striving to keep compliance with bitcoin core as the default behavior.

[fametrano]

please rebase and check that all new 5 GitHib actions succed

[sourcery-ai[bot]]

Sourcery Code Quality Report

❌  Merging this PR will decrease code quality in the affected files by 1.35%.

Quality metrics	Before	After	Change
Complexity	1.96 ⭐	2.08 ⭐	0.12 👎
Method Length	85.88 🙂	93.24 🙂	7.36 👎
Working memory	7.51 🙂	7.71 🙂	0.20 👎
Quality	68.86% 🙂	67.51% 🙂	-1.35% 👎

Other metrics	Before	After	Change
Lines	1728	2205	477

Changed files	Quality Before	Quality After	Quality Change
btclib/bip32/key_origin.py	88.58% ⭐	88.02% ⭐	-0.56% 👎
btclib/psbt/__init__.py	80.59% ⭐	69.66% 🙂	-10.93% 👎
btclib/psbt/psbt.py	60.51% 🙂	61.12% 🙂	0.61% 👍
btclib/tx/__init__.py	94.23% ⭐	91.08% ⭐	-3.15% 👎
btclib/tx/tx.py	80.51% ⭐	74.62% 🙂	-5.89% 👎
tests/bip32/test_key_origin.py	68.39% 🙂	68.53% 🙂	0.14% 👍
tests/psbt/test_psbt.py	74.03% 🙂	71.98% 🙂	-2.05% 👎
tests/tx/test_tx.py	60.10% 🙂	59.35% 🙂	-0.75% 👎

Here are some functions in these files that still need a tune-up:

File	Function	Complexity	Length	Working Memory	Quality	Recommendation
btclib/psbt/psbt.py	Psbt.assert_signable	22 😞	163 😞	11 😞	40.42% 😞	Refactor to reduce nesting. Try splitting into smaller methods. Extract out complex expressions
btclib/psbt/psbt.py	Psbt.parse	10 🙂	188 😞	15 😞	41.77% 😞	Try splitting into smaller methods. Extract out complex expressions
tests/tx/test_tx.py	test_tx	0 ⭐	697 ⛔	12 😞	46.17% 😞	Try splitting into smaller methods. Extract out complex expressions
btclib/psbt/psbt.py	join_psbts	5 ⭐	158 😞	16 ⛔	48.14% 😞	Try splitting into smaller methods. Extract out complex expressions
tests/psbt/test_psbt.py	test_vectors_bip174	5 ⭐	208 ⛔	12 😞	49.23% 😞	Try splitting into smaller methods. Extract out complex expressions

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