Fix: Support all hash types in Prefix.sum()

[sumanjeet0012]

Closes #64

Description

This PR addresses the limitation where Prefix.sum() and V1Builder.sum() only supported sha2-256 and sha2-512 by hardcoding hashlib.sha256/512. The hashing logic now correctly delegates to multihash.sum() which supports all registered hash functions in python-multihash.

Changes

• Updated Prefix.sum() in cid/prefix.py to use multihash.sum() instead of manually invoking hashlib and checking mh_type.
• Updated V0Builder.sum() and V1Builder.sum() in cid/builder.py similarly to delegate to multihash.sum().
• Removed unnecessary hashlib imports from both modules.
• Added tests in tests/test_prefix.py to ensure various multihash types like sha3-256, blake2b-256, and identity are supported.

[seetadev]

Thanks for another excellent contribution, @sumanjeet0012.

This is a clean and well-designed improvement that makes py-cid more flexible and better aligned with the multiformats ecosystem. Instead of hardcoding support for only sha2-256 and sha2-512, delegating the hashing logic to multihash.sum() is the right abstraction, allowing Prefix.sum() and the builder APIs to automatically support the full set of registered multihash algorithms.

This not only simplifies the implementation by removing duplicate hashing logic, but also ensures future compatibility as additional hash functions are supported by python-multihash.

I also appreciate the accompanying test coverage for algorithms such as sha3-256, blake2b-256, and identity, which gives good confidence that the new behavior works as intended and helps prevent regressions going forward. The implementation is concise, the unnecessary imports have been removed, and the full CI matrix is passing successfully across all supported environments.

Overall, this is a valuable improvement that enhances standards compliance while reducing maintenance overhead.

LGTM, thanks for the thoughtful contribution and happy to merge.

CCing @acul71, @johannamoran and @mishmosh.