Add type-specific key creation convenience methods

[mtmk]

Summary

The Go implementation provides type-specific factory functions for convenience:

• CreateUser(), CreateAccount(), CreateServer(), CreateCluster(), CreateOperator()
• CreateCurveKeys() / CreateCurveKeysWithRand()

These are sugar over CreatePair(prefix) but improve readability and discoverability.

Approach

Add static methods to KeyPair. Purely additive — no existing API changes needed.