Spending Policy Required: External error on tx builder finish

[BitcoinZavior]

Describe the bug

Encountering a Spending policy required: External error when attempting to build a transaction using a multi-sig descriptor. Currently, bdk-ffi does not provide a policy_path() in txBuilder, which is necessary for specifying the correct policy path during transaction creation.

Proposed Solution:

• Expose policies() from the wallet to retrieve available spending policies.
• Add support for policy_path() in txBuilder to allow specifying the correct policy path when building the transaction.

These changes will ensure that the correct branch of a script is selected when creating transactions, preventing ambiguity issues.

Use Case:
An example where the policy_path is needed is with the following descriptor:

wsh(thresh(2,pk(A),sj:and_v(v:pk(B),n:older(6)),snj:and_v(v:pk(C),after(630000)))),
which is derived from the miniscript policy:

thresh(2,pk(A),and(pk(B),older(6)),and(pk(C),after(630000))).
In this case, the script can be satisfied in multiple ways. Without specifying the policy_path, it becomes ambiguous which spending path should be used for the transaction, leading to the error.

[BitcoinZavior]

@afilini issue raised for LtbLightning/bdk-flutter#144

[reez]

Closing due to #629 being merged now-