Merge bitcoindevkit#205: feature: Add Caravan wallet format import/export support

1dbe85a docs: add round-trip limitation notice for Caravan export/import (vkprogrammer-001)
d8596e6 Fix: improve derivation path handling and add tests (vkprogrammer-001)
1068afd Fix: Replace string manipulation with stronger Bitcoin types in Caravan export (vkprogrammer-001)
2e920a2 Add Caravan wallet format support (vkprogrammer-001)

Pull request description:

  ### Description
  This PR implements support for importing and exporting wallets in the Caravan wallet format as requested in issue bitcoindevkit#5.

  ### Changes

  - Added `CaravanExport` struct to handle Caravan wallet format
  - Implemented methods to export a wallet to Caravan JSON format
  - Added functionality to import Caravan wallet configs and convert to BDK descriptors
  - Added documentation with usage examples
  - Added comprehensive test coverage for all supported multisig types (P2SH, P2WSH, P2SH-P2WSH)

  ### Implementation Details
  - All code is contained in the wallet/export.rs module alongside the existing FullyNoded export functionality
  - Support for different address types required by Caravan (P2SH, P2WSH, P2SH-P2WSH)
  - Handles the mapping between BDK descriptor format and Caravan's JSON configuration format

  ### Testing
  The implementation includes tests for:
  - Exporting BDK wallets to Caravan format
  - Importing Caravan configurations to BDK wallets
  - All supported address types (P2SH, P2WSH, P2SH-P2WSH)
  - Network detection (mainnet/testnet)

  ### 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

  Closes bitcoindevkit#5

ACKs for top commit:
  ValuedMammal:
    reACK 1dbe85a

Tree-SHA512: ef144089f8b1b3de18d1c6c518259f6ad7132b88cf52573befba557221b3302f01d9197a79c7918411806d9b59047ca24ec03f617acbad3980de3c185905c38f

Author: ValuedMammal