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/querier.go

@@ -169,7 +169,7 @@ type sqlcQuerier interface {
 	DeleteTask(ctx context.Context, arg DeleteTaskParams) (uuid.UUID, error)
 	DeleteUserChatCompactionThreshold(ctx context.Context, arg DeleteUserChatCompactionThresholdParams) error
 	DeleteUserChatProviderKey(ctx context.Context, arg DeleteUserChatProviderKeyParams) error
-	DeleteUserSecretByUserIDAndName(ctx context.Context, arg DeleteUserSecretByUserIDAndNameParams) error
+	DeleteUserSecretByUserIDAndName(ctx context.Context, arg DeleteUserSecretByUserIDAndNameParams) (int64, error)
 	DeleteWebpushSubscriptionByUserIDAndEndpoint(ctx context.Context, arg DeleteWebpushSubscriptionByUserIDAndEndpointParams) error
 	DeleteWebpushSubscriptions(ctx context.Context, ids []uuid.UUID) error
 	DeleteWorkspaceACLByID(ctx context.Context, id uuid.UUID) error