Implement basic spending policy APIs

[notmandatory]

Description

Add or update APIs in bdk-ffi to support basic spending policy features. Also test the new APIs by updating bdk-kotlin or bdk-swift to use the new and updated APIs. See "Spending Policy Demo" for bdk-cli based example usage.

APIs include:

1. Giving a wallet, return Policy structure (as Dictionary?) for "external" and "internal" descriptors.
2. When creating a transaction PSBT, specify the "external policy" to use

Expected Outcomes

• Update bdk.udl and lib.rs with functions and structs as needed
• Update bdk-kotlin or bdk-swift and add tests to cover new spending policy functionality

Resources

• BDK Website
• BDK github repo
• BDK Discord
• uniffi-rs

Skills Required

• Experience with git. Guide
• Basic familiarity with rust. First seven chapters of the book
• Basic familiarity with Kotlin or Swift
• Familiarity with bitcoin descriptors and miniscript policies

Mentor(s)

@artfuldev @notmandatory

Difficulty

Medium

Competency Test (optional)

• Install rust, compile bdk-ffi and build and test bdk-swift or bdk-kotlin.
• Read through the BDK docs.
• Use bdk-cli to walk through the "Spending Policy Demo"
• Install and run example bdk-kotlin or bdk-swift example wallet.
• Familiarity with basic rust, should be able to write basic structs and functions.

[thunderbiscuit]

This issue might need an update, as I think the Policy module in bdk will be replaced by the policy module in miniscript.

[notmandatory]

For 1.0 I don't expect the policy features in bdk to change much. The current plan is to introduce the new "planning module" features in the 2.0 milestone.

[LLFourn]

I would not implement policy.rs api because of this change FWIW.

[thunderbiscuit]

This feature was requested and added in the 1.0 release by #612.