Update build_tx to support policy path specification.

[i5hi]

Timelocked custodian specifically requires specifying policy path in the transaction build stage.

Keep the requirement generic and as expected by BDK.

Create a generalized input of policy_id and policy_path as a stringified JSON. Structured PolicyPath will be as follows.

PolicyPath{ id: String, path: Vec }

It will impl a conversion to a BTreeMap as required by BDK.

impl PolicyPath{ to_btree_map(&self)->BTreeMap<String,Vec{} }