feat(selector): allow multiple change sources with ChangeDescriptor

[nymius]

Description

Previously, the change_descriptor field only accepted a definite descriptor to derive the change output script pubkey. This worked for most cases but failed for scenarios - such as silent payments - where a definite descriptor is not yet available.

To address this, introduce the ChangeDescriptor enum, which supports:

• The standard case of providing a definite descriptor.
• An alternative where the script pubkey and its maximum satisfaction weight can be specified directly.

This makes change output generation flexible enough to handle both standard and exceptional workflows.

Fixes #17

All Submissions:

• I've signed all my commits
• I followed the conventional commit guidelines
• I ran cargo +nigthly fmt before committing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

[ValuedMammal]

ACK aadeca5

[evanlinjin]

ApproachNACK

There is a problem with the current approach: SelectorParams has two independent fields that can both specify the change output's satisfaction weight, and they can disagree.

• change_script: ScriptSource — when this is the Descriptor variant, the satisfaction weight is derivable from the descriptor via max_weight_to_satisfy.
• change_weight: DrainWeights — this also encodes the satisfaction weight (the spend cost of the change output).

The caller is responsible for keeping these in sync, but nothing enforces that. If change_script is a Descriptor and the caller provides a DrainWeights with a different satisfaction weight, we silently use the wrong value.