perf: add index to improve the GetWorkspaceAgentByInstanceID query performance (#20936)

## Context

GetWorkspaceAgentByInstanceID has a suboptimal plan. Even though it is
designed to fetch a small subset of records, there are no corresponding
indexes and that query results in full table scan:

Query:

```
SELECT id, auth_instance_id FROM workspace_agents
where auth_instance_id='i-013c2b96b6441648a' and deleted=FALSE;
```

Plan:

```
------------------------------------------------------------------------------------------------------------------
 Seq Scan on workspace_agents  (cost=0.00..222325.48 rows=2 width=36) (actual time=0.012..234.152 rows=4 loops=1)
   Filter: ((NOT deleted) AND ((auth_instance_id)::text = 'i-013c2b96b6441648a'::text))
   Rows Removed by Filter: 302276
 Planning Time: 0.173 ms
 Execution Time: 234.169 ms
```

After adding the index, the plan improves drastically.

Updated plan:

```
 Bitmap Heap Scan on workspace_agents  (cost=4.44..12.32 rows=2 width=36) (actual time=0.019..0.019 rows=0 loops=1)
   Recheck Cond: (((auth_instance_id)::text = 'i-013c2b96b6441648a'::text) AND (NOT deleted))
   ->  Bitmap Index Scan on workspace_agents_auth_instance_id_deleted_idx  (cost=0.00..4.44 rows=2 width=0) (actual time=0.013..0.014 rows=0 loops=1)
         Index Cond: (((auth_instance_id)::text = 'i-013c2b96b6441648a'::text) AND (deleted = false))
 Planning Time: 0.388 ms
 Execution Time: 0.044 ms
```

## Changes

* add an index to optimize this query

## Testing

* ran the queries manually against prod and test DBs
* ran `./scripts/develop.sh`, connected to the local PostgreSQL
instance, inspected the indexes to make sure new index is there:

```
Indexes:
    "workspace_agents_pkey" PRIMARY KEY, btree (id)
    // NEW INDEX CREATED SUCCESSFULLY  [comment is mine]
    "workspace_agents_auth_instance_id_deleted_idx" btree (auth_instance_id, deleted)
    "workspace_agents_auth_token_idx" btree (auth_token)
    "workspace_agents_resource_id_idx" btree (resource_id)
```

---------

Signed-off-by: Danny Kopping <danny@coder.com>
Co-authored-by: Danny Kopping <danny@coder.com>

Author: mykyta-protsenko

coderd/database/dump.sql

@@ -0,0 +1 @@
+DROP INDEX IF EXISTS public.workspace_agents_auth_instance_id_deleted_idx;

coderd/database/migrations/000401_add_workspace_agents_index.down.sql

@@ -0,0 +1 @@
+CREATE INDEX IF NOT EXISTS workspace_agents_auth_instance_id_deleted_idx ON public.workspace_agents (auth_instance_id, deleted);