feat: add REST API handlers and client methods for user secrets (#24107)

Add the five REST endpoints for managing user secrets, SDK client
methods, and handler tests.

Endpoints:
- `POST /api/v2/users/{user}/secrets`
- `GET /api/v2/users/{user}/secrets`
- `GET /api/v2/users/{user}/secrets/{name}`
- `PATCH /api/v2/users/{user}/secrets/{name}`
- `DELETE /api/v2/users/{user}/secrets/{name}`

Routes are registered under the existing `/{user}` group with
`ExtractUserParam`. The delete query was changed from `:exec` to
`:execrows` so the handler can distinguish "not found" from success
(DELETE with `:exec` silently returns nil for zero affected rows).

coderd/database/queries/user_secrets.sql

@@ -56,6 +56,6 @@ SET
 WHERE user_id = @user_id AND name = @name
 RETURNING *;
 
--- name: DeleteUserSecretByUserIDAndName :exec
+-- name: DeleteUserSecretByUserIDAndName :execrows
 DELETE FROM user_secrets
 WHERE user_id = @user_id AND name = @name;