BitGo / rust-bitcoin

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feat(crypto): add FORKID sighash support for altcoins

Add support for Bitcoin Cash/Ecash, Bitcoin Gold and other FORKID-based
coins by implementing BIP143-style sighash algorithm with fork ID
support. This allows:

1. Handling transactions with SIGHASH_FORKID flag (0x40)
2. Computing correct FORKID sighashes with (fork_id << 8 | sighash_type)
3. Signing PSBTs with BCH/BTG forkid support via new sign_forkid method

The implementation preserves raw sighash types in Signature.sighash_type
field while providing methods to extract the standard base type.
PSBT deserialization now accepts any sighash byte for better compatibility.

Tests include:
- BCH test vectors for FORKID sighash computation
- BitGo cross-chain PSBT fixtures for Bitcoin and altcoins

Issue: BTC-2656

Co-authored-by: llm-git <llm-git@ttll.de>

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