feat: add async counterparts for public encrypt/decrypt methods

[bdesoky]

Ticket: WCN-284

This pull request introduces async versions of several cryptographic and wallet management methods in the BitGo SDK, aiming to modernize the codebase and improve compatibility with v2 encryption. It also updates and expands the test suites to cover these new async methods, ensuring both backward compatibility and robust error handling. The most significant changes are grouped below:

Async Cryptographic Methods and API Enhancements

• Added decryptKeysAsync, an async method for decrypting multiple wallet keys with v2 encryption support, and updated related documentation and error handling.
• Added handleTokenIssuanceAsync, an async method for securely handling authentication responses and token issuance, and refactored internal usage to prefer the async version. [1] [2] [3]
• Added splitSecretAsync and reconstituteSecretAsync for Shamir Secret Sharing, providing async support for splitting and reconstructing secrets, with improved input validation and error messages. [1] [2]
• Updated keychain addition and encryption flows to use encryptAsync for private key encryption, ensuring consistency with async cryptography.
• Marked several legacy sync methods for deprecation in favor of their async counterparts, preparing for a future transition to v2 encryption by default. [1] [2] [3]

Test Suite Updates and Coverage

• Refactored BitGo prototype method tests to use async versions (splitSecretAsync, reconstituteSecretAsync, verifyShardsAsync) and added comprehensive new tests for all async Shamir Secret Sharing flows, including error cases and success scenarios. [1] [2]
• Added new tests for handleTokenIssuanceAsync and updated authentication response handling tests to use async methods.
• Introduced async test coverage for wallet methods such as encryptPrvForUserAsync and downloadKeycardAsync, including error handling for Node.js environments and correct output validation. [1] [2]

These changes modernize the SDK's cryptographic workflows, improve test coverage, and lay the groundwork for a full migration to async and v2 encryption methods.

[linear-code]

WCN-284