Replace bdk_tmp_plan with miniscript planning api

[LLFourn]

@danielabrozzoni looks like she's gotten rust-bitcoin/rust-miniscript#481 (comment) to a good place. It would be helpful if we could replace what we currently use for this role with that PR.

There was a very outdated attempt at doing this by @afilini here: https://github.com/LLFourn/bdk_core_staging/pull/66/files#diff-c14cb78dacd186a4264416678c250c834363aadab6cb8d1657709ffc970ca9ffR469

This has the important sign_with_keymap whose logic needs to go somewhere probably but maybe there might be some state of the art in rust-bitcoin or rust-miniscript to help with the signing part.

The goal of this PR is to make a github dependency on that PR and see how we can use it in the existing examples that use bdk_tmp_plan.