Add support for protected roles

[joelynch]

Extends the existing protected-entity mechanism (the per-entity Protected flag plus the PROTECTED_ACCESS_MANAGEMENT global privilege, SQL keyword PROTECTED) to ROLE entities. A role marked PROTECTED can only be created, altered, renamed, replaced, dropped, moved between storages, or have its grants changed by a principal holding PROTECTED_ACCESS_MANAGEMENT. This lets service-managed roles exist as real SQL roles that ordinary cluster users cannot remove or tamper with.

Role-specific changes:

• Role: add a protected_flag member, an isProtected() override, and include the flag in equal().
• ASTCreateRoleQuery / ParserCreateRoleQuery: carry and parse the PROTECTED keyword on CREATE ROLE and [NOT] PROTECTED on ALTER ROLE, and round-trip it in formatImpl.
• InterpreterCreateRoleQuery: require PROTECTED_ACCESS_MANAGEMENT when the statement sets PROTECTED; before any ON CLUSTER dispatch, read the existing role (ALTER and CREATE OR REPLACE) and require the privilege when the existing or new role is protected; re-check atomically via the insertOrReplace CheckFunc as defense-in-depth.
• InterpreterShowCreateAccessEntityQuery: emit PROTECTED so the flag survives SHOW CREATE, ZooKeeper replication and backup/restore.

The storage-layer CheckFunc plumbing and the DROP, GRANT/REVOKE and MOVE interpreters are entity-agnostic and already key off isProtected(), so DROP ROLE, GRANT/REVOKE and MOVE on a protected role are covered without further changes.

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Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

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Documentation entry for user-facing changes

• Documentation is written (mandatory for new features)