Send GPU config string

To understand whether users are enabling GPUs in a CI runner with a Docker executor, we now send the config string to know whether they are enabled.

The /api/v4/jobs/prequest payload now has a config Hash:

{
   "info":{
      "name":"gitlab-runner",
      "version":"13.11.0~beta.109.ga917c299",
      "revision":"a917c299",
      "platform":"darwin",
      "architecture":"amd64",
      "executor":"docker",
      "shell":"bash",
      "features":{
         "variables":true,
         "image":true,
         "services":true,
         "artifacts":true,
         "cache":true,
         "shared":false,
         "upload_multiple_artifacts":true,
         "upload_raw_artifacts":true,
         "session":true,
         "terminal":true,
         "refspecs":true,
         "masking":true,
         "proxy":false,
         "raw_variables":true,
         "artifacts_exclude":true,
         "multi_build_steps":true,
         "trace_reset":true,
         "trace_checksum":true,
         "trace_size":true,
         "vault_secrets":true,
         "cancelable":true,
         "return_exit_code":true
      },
      "config":{
         "gpus":"all"
      }
   },
   "token":"REDACTED"
}

This requires gitlab!57953 (merged) to store on the GitLab side.

Relates to:

1. gitlab#27141 (comment 517145781).
2. gitlab#322528 (closed)