doc: add support for SHA3 hashing algorithm

[kailash360]

Summary

Adds a Milvus Enhancement Proposal (MEP) documenting SHA-3 (SHA3-256) support as a configurable alternative to SHA-256 for internal hashing operations in Milvus.

Related
Implementation PR: milvus-io/milvus#48913
Issue: milvus-io/milvus#25970

What this doc covers

• Motivation for making the hash algorithm configurable for compliance-sensitive deployments
• Design of the ComputeHash abstraction in internal/util/crypto
• New config parameter common.security.hashAlgorithm (default: sha256, supported: sha3)
• Compatibility analysis — existing deployments unaffected, SHA-256 remains the default
• Test plan covering unit, paramtable, and telemetry-level tests

[sre-ci-robot]

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[sre-ci-robot]

Welcome @kailash360! It looks like this is your first PR to milvus-io/milvus-design-docs 🎉

[kailash360]

@zwd1208 @sunby
Kindly review the design doc and let me know if any changes are needed.