Create miniscript `plan` example

[ValuedMammal]

Work is ongoing on bitcoindevkit/bdk#1559

[LLFourn]

The impact of the planning module to BDK is mostly internal. It was designed to remove the policy.rs module and replace it. The key thing is that the weights of the planning module can be used to inform coin selection rather than using the max satisfaction weight. The example in the PR is glorious but maybe premature since it's updating the PSBT with the plan manually where as we'd actually like to do this doing transaction creation and not leave it up to the user.