feature: Add Caravan wallet format import/export support

[vkprogrammer-001]

Description

This PR implements support for importing and exporting wallets in the Caravan wallet format as requested in issue #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:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

Closes #5

[coveralls]

Pull Request Test Coverage Report for Build 14361745616

Details

• 260 of 278 (93.53%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 86.75%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
wallet/src/wallet/export.rs	260	278	93.53%

Totals
Change from base Build 14270666930:	0.2%
Covered Lines:	7529
Relevant Lines:	8679

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💛 - Coveralls