proxy: split out the console-redirect backend flow

[conradludgate]

removes the ConsoleRedirect backend from the main auth::Backends enum, copy-paste the existing crate::proxy::task_main structure to use the ConsoleRedirectBackend exclusively.

This makes the logic a bit simpler at the cost of some fairly trivial code duplication.

[github-actions]

5058 tests run: 4872 passed, 0 failed, 186 skipped (full report)

---

Flaky tests (5)

Postgres 16

• test_ondemand_wal_download_in_replication_slot_funcs: release-x86-64
• test_location_conf_churn[3]: release-x86-64
• test_pg_regress[4]: release-x86-64
• test_storage_controller_heartbeats[failure4]: release-x86-64

Postgres 15

• test_subscriber_restart: release-x86-64

Code coverage* (full report)

• functions: 31.3% (7492 of 23914 functions)
• lines: 49.5% (60142 of 121583 lines)

* collected from Rust tests only

---

_{The comment gets automatically updated with the latest test results
74e013d at 2024-10-03T21:45:26.864Z :recycle:}