validator: enable authorization by default

[QuerthDP]

Enable ScyllaDB authorization (PasswordAuthenticator + CassandraAuthorizer) by default across the validator harness, so the e2e suite exercises the same reduced-privilege setup used in production (Cloud) deployments
instead of full/unauthenticated access.

The default ScyllaDB cluster config now enables auth, and the Vector Store connects using a dedicated role granted only VECTOR_SEARCH_INDEXING and TEXT_SEARCH_INDEXING (mirroring the fine-grained indexing permissions used in Cloud rather than full SELECT). The validator's own test-driver CQL sessions keep connecting as an internal superuser so existing test setup/assertions are unaffected.

Dedicated auth tests already manage their own isolated superuser/role setup and are unaffected, since the default bootstrap only runs when a cluster was started with the shared default auth config.

Fixes: VECTOR-758

[Copilot]

Pull request overview

Enables authenticated, reduced-privilege ScyllaDB access throughout the validator harness.

Changes:

• Adds default superuser bootstrap and restricted Vector Store role.
• Authenticates validator CQL sessions as superuser.
• Preserves authentication in Alternator and HA configurations.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
crates/validator/src/common.rs	Adds default authentication and role setup.
crates/validator/src/high_availability.rs	Enables authentication for the HA test.
crates/validator/src/alternator/mod.rs	Retains default auth unless explicitly overridden.

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[QuerthDP]

Changelog:

• added use_default_auth bool param to the init_with_config
• deduplicated the SUPERUSER setup from auth.rs and alternator/auth.rs