Basic taproot BIP32 derivation & keyspend support

[kdmukai]

• Adds basic support for the BIP-371 Taproot psbt field PSBT_IN_TAP_BIP32_DERIVATION (0x16) to properly parse bip32 derivations in Taproot psbts.
• Only supports the root ("internal") key; does nothing if the psbt is expecting to be signed by a key within a leaf.
• Also adds PSBT_IN_TAP_INTERNAL_KEY (0x17), though I believe it won't be needed until keys within leaves are supported (internal key will match the PSBT_IN_TAP_BIP32_DERIVATION pubkey when num_leaf_hashes is zero).
• Adds the matching PSBT_OUT_TAP_INTERNAL_KEY (0x05) and PSBT_OUT_TAP_BIP32_DERIVATION (0x07) fields to outputs.
• Stores the data in InputScope.taproot_bip32_derivations which is a tuple: ([leaf_hashes], DerivationPath). Same for OutputScope.taproot_bip32_derivations.
• Stores the internal key in InputScope.taproot_internal_key and OutputScope.taproot_internal_key.
• Signing is backwards-compatible with "legacy" Taproot implementation for psbts that do not have the PSBT_IN_TAP_BIP32_DERIVATION field (i.e. they populate 0x06/InputScope.bip32_derivations).
• Updates Taproot psbt signing to expect X-only pubkeys.
• Updates test cases to test that taproot_bip32_derivations were populated and adds fixes so that the X-only pubkeys expectation passes.

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I will also try to make a PR to update Specter Desktop to output the 0x16 and 0x17 BIP-371 fields. Because of the backwards-compatibility here, a Specter-DIY release can come first and then update Specter Desktop with these fields.

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TODO:
Update test_tapoot.TaprootTest.test_sign_verify to use a psbt that includes the PSBT_IN_TAP_BIP32_DERIVATION field rather than the "legacy" psbt approach.

TODO:
test_psbtview.PSBTViewTest is failing on test_sign:

FAIL: test_sign (tests.test_psbtview.PSBTTest)
Test if we can sign psbtview and get the same as from signing psbt
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Traceback (most recent call last):
  File "/Users/kdmukai/dev/embit/tests/tests/test_psbtview.py", line 113, in test_sign
    self.assertEqual(ser.getvalue(), psbt.serialize())
AssertionError: b'psb[844 chars]0\x00"\x02\x02\xa9\x81\xb3?~\xb9*\n\xcf77P\x92[890 chars]\x00' != b'psb[844 chars]0\x00\x01\x01\x1f\x80\x96\x98\x00\x00\x00\x00\[2246 chars]\x00'

This failure pre-dates these changes so something was either already broken or something is off with my local setup.

[kdmukai]

FYI - still making improvements and tweaks so this PR should be considered DRAFT (but I don't seem to have permissions to revert the PR status to DRAFT).

[stepansnigirev]

Converted to draft

[stepansnigirev]

PSBT parsing should not fail if the library can't sign - maybe it's not used for signing at all. You can raise when signing, but I'd better ignore inputs we don't know how to sign.
For example, when you try to sign PSBT transaction with a wrong key it doesn't raise an error, it just returns 0 as a number of added signatures. I think for tap leafs we should do the same.