Provide a way to broadcast raw transaction for LDK compatibility

[thunderbiscuit]

The LDK library requires a way to broadcast a raw transaction.

To enable this in the language bindings, we'd need to either overload the broadcast() function, enabling it to take raw bytes as an argument (currently takes a PSBT), or provide a utility function that would transform a raw transaction into a PSBT (not sure if that can really be done or how to do it yet, will look into it).

[notmandatory]

I think PartiallySignedTransaction::consensus_decode() will do what we need.

[notmandatory]

This issue came up again as a request on discord.

[notmandatory]

I think the cleanest way to make this work is:

1. Create a Transaction struct, with a default constructor that takes serialized transaction bytes (as provided by LDK).
2. Add a method to PartiallySignedTransaction called extract_tx() that returns a Transaction.
3. Change the Blockchain.broadcast() method to take a Transaction instead of a PartiallySignedBitcoinTransaction.

This will also keep the language bindings API more inline with the core BDK Rust API.