bip-0046: Address Scheme for Timelocked Fidelity Bonds

This is a continuation of @chris-belcher's PR#1341 titled "Derivation scheme for timelocked address fidelity bond based accounts", as Chris will quite possibly not be able to progress this any further. Please see comments in the PR, especially https://github.com/bitcoin/bips/pull/1341#issuecomment-2112742401:

It will probably be the easiest to open a new PR that supersedes this one.

TBD (from https://github.com/bitcoin/bips/pull/1341#pullrequestreview-2055394826):

• [x] Assign BIP number (https://github.com/bitcoin/bips/pull/1341#issuecomment-2112743358)
• [x] Copyright section
• [x] Backwards Compatibility section
• [x] Rationale section

As soon as all missing points are addressed, #1341 can be closed and this PR un-drafted. Happy for any and all feedback, particularly when it comes to finalizing it. :pray: