Add Psbt::sign API to Psbt type

[ItoroD]

Describe the enhancement

Psbt::sign is yet to exposed in our Psbt type. This allows you to sign a Psbt be simply calling the sign method from rust-bitcoin Psbt::sign on the psbt you would like to sign.

Use case

• To sign a psbt

Additional context

In preparation for bdk-wallet 3.0 migration, Signers will be removed from bdk-wallet as described here. Having this feature serves as a replacement and enhancement of how we sign psbt's.

Using the sign method requires as parameters a type that impelements GetKey and a Secp256k1. For GetKey it is important to note that types like KeyMap and DescriptorSecretKey have implemented GetKey in rust-miniscript and there might be no need to create any extra types here in FFI. KeyMap and DescriptorSecretKey are existing types in FFI which will have access to implemented GetKey;

• once bdk-wallet updates to rust-miniscript version that has GetKey implementation for those types,
• and then bdk-ffi updates to updated bdk-wallet version.

Once psbt sign method is exposed, and the above conditions fulfilled, using psbt.sign will be as simple as passing the Descriptor's KeyMap as param. Below is the PR to watch/wait for, for this issue to be completed.

• PR: remove signers

Note: In order to see how sign from psbt will work here in FFI even before all above conditions are satisfied, one can use a SignerWrapper from testutils like in PR: remove signers