BIP0174 PSBT Support

Create a new PartiallySignedTransaction class that allows creation, updating and signing of transactions in the PSBT BIP0174 format.

• Create classes for the underlying PSBT data structure: PSBTKeyValue, PSBTMap as Writable classes.
• Create abstract PSBT class for overall data structure: PartiallySignedTransaction. Include interface for updating input/output data, signing, merging, finalizing and obtaining the complete transaction.
• Add write method to PartiallySignedTransaction.
• Create PartiallySignedTransactionV1 to represent the BIP0174 PSBT which is technically v0 but often regarded as v1. Include a create named constructor.
• Implement update methods.
• Implement signing.
• Implement merging.
• Implement finalization.
• Implement transaction construction.
• Add a fromReader factory constructor to PartiallySignedTransaction to check for and return the relevant sub-class: only PartiallySignedTransactionV1 to begin with.
• Add all test vectors for BIP0174.

Start by reading the BIP0174 specification and the repository's existing transaction and Writable abstractions. Map the requested PSBTKeyValue, PSBTMap, PartiallySignedTransaction, and PartiallySignedTransactionV1 classes before implementing their update, signing, merging, finalization, and construction interfaces. Use the BIP0174 test vectors to define completion.