Model MySQL CREATE USER statements

[minleejae]

Summary

• parse MySQL CREATE USER into a dedicated statement AST
• expose accounts, hosts, authentication plugins, credentials, and random-password modes as structured values
• support multiple accounts and IF NOT EXISTS while preserving parse/deparse behavior

Validation

• ./gradlew spotlessApply test spotlessCheck checkstyleMain checkstyleTest
• executed representative account/host/password forms against MySQL 8.4

[manticore-projects]

Please resolve conflicts.

[minleejae]

@manticore-projects The merge conflicts have been resolved, and the branch has been rebased onto the latest master. This PR is ready for review again. Thank you.