perf(db): Add indexes on foreign key to improve delete performance

This adds indexes for a few categories:

1. Foreign keys on the session table. These are set to `null` when the
   referenced row is deleted. These indexes will help to more
   efficiently set these `null` values in the case where a target is
   deleted, or and auth token is deleted.
2. Foreign keys from other tables to the session table. These are either
   set to `null` or cascade deleted when a session is deleted. These
   indexes help with these update/deletes when a session is deleted.
3. A multi-column index on session_state. This helps with the query that
   is used to delete all terminated sessions that have been terminated
   for over an hour.

(cherry picked from commit 263a96e9dd)

internal/db/schema/migrations/oss/postgres/91/02_indexes_fk_delete.up.sql

@@ -0,0 +1,46 @@
+-- Copyright (c) HashiCorp, Inc.
+-- SPDX-License-Identifier: BUSL-1.1
+
+begin;
+  -- Index to help when setting target_id to null on a session
+  -- when the corresponding target is deleted.
+  drop index if exists session_target_id_ix;
+  create index session_target_id_ix
+            on session (target_id);
+
+  -- Index to help when setting auth_token_id to null on a session
+  -- when the corresponding auth_token is deleted.
+  drop index if exists session_auth_token_id_ix;
+  create index session_auth_token_id_ix
+            on session (auth_token_id);
+
+  -- Index to help when setting session_id to null on a credential_vault_credential
+  -- when the corresponding session is deleted.
+  drop index if exists credential_vault_credential_session_id_ix;
+  create index credential_vault_credential_session_id_ix
+            on credential_vault_credential (session_id);
+
+  -- Index to help delete cascade of session_worker_protocol
+  -- when the corresponding session is deleted.
+  drop index if exists session_worker_protocol_session_id_ix;
+  create index session_worker_protocol_session_id_ix
+            on session_worker_protocol (session_id);
+
+  -- Index to help when setting session_id to null on recording_connection
+  -- when the corresponding session is deleted.
+  drop index if exists recording_connection_session_id_ix;
+  create index recording_connection_session_id_ix
+            on recording_connection (session_id);
+
+  -- Index to help delete of terminated sessions.
+  drop index if exists session_state_state_start_time_ix;
+  create index session_state_state_terminated_start_time_ix
+            on session_state (state, start_time)
+         where state = 'terminated';
+
+  analyze session,
+          credential_vault_credential,
+          session_worker_protocol,
+          recording_connection,
+          session_state;
+commit;