Store CI runner config data

With gitlab-runner!2848 (merged), we will now be sending some information about the runner config. Currently, this is only limited to the gpus string, but it can be extended to cover more.

gitlabhq_development=# select config, created_at, updated_at, contacted_at from ci_runners where config != '{}';
     config      |         created_at         | updated_at |        contacted_at
-----------------+----------------------------+------------+----------------------------
 {"gpus": "all"} | 2020-02-18 21:52:11.882439 |            | 2021-05-14 22:07:43.109804

Migration

exec ALTER TABLE "ci_runners" ADD "config" jsonb DEFAULT '{}' NOT NULL;
Session: 4228
The query has been executed. Duration: 10.352 ms

Rollback

exec ALTER TABLE "ci_runners" DROP COLUMN "config";
Session: 4228
The query has been executed. Duration: 4.939 ms