Merge bitcoindevkit/bdk#1324: [chain] Make KeychainTxOutIndex more range based

Merge bitcoindevkit/bdk#1324: [chain] Make KeychainTxOutIndex more range based

fac228337c79e1a7c406500c177e049e7f9f5da7 feat(chain)!: make `KeychainTxOutIndex` more range based (LLFourn)

Pull request description:

  KeychainTxOut index should try and avoid "all" kind of queries. There may be subranges of interest. If the user wants "all" they can just query "..".

  The ideas is that KeychainTxOutIndex should be designed to be able to incorporate many unrelated keychains that can be managed in the same index. We should be able to see the "net_value" of a transaction to a specific subrange. e.g. imagine a collaborative custody service that manages all their user descriptors inside the same `KeychainTxOutIndex`. One user in their service may pay another so when you are analyzing how much a transaction is spending for a particular user you need to do analyze a particular sub-range.

  ### Notes to the reviewers

  - I didn't change `unused_spks` to follow this rule because I want to delete that method some time in the future. `unused_spks` is being used in the examples for syncing but it shouldn't be (the discussion as to why will probably surface in #1194).
  - I haven't applied this reasoning to the methods that return `BTreeMap`s e.g. `all_unbounded_spk_iters`. It probably should be but I haven't made up my mind yet.

  This probably belongs after #1194

  ### Changelog notice

  - `KeychainTxOutIndex` methods modified to take ranges of keychains instead.

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

  #### New Features:

  * [x] I've added tests for the new feature
  * [x] I've added docs for the new feature

ACKs for top commit:
  evanlinjin:
    ACK fac228337c79e1a7c406500c177e049e7f9f5da7

Tree-SHA512: ec1e75f19d79f71de4b6d7748ef6da076ca92c2f3fd07e0f0dc88e091bf80c61268880ef78be4bed5e0dbab2572e22028f868f33e68a67d47813195d38d78ba5