Persist job inputs spec in Ci::Build options field

We need to store the definitions for a job's CI job inputs in the options field on ci_builds. The content from this field will later be stored in ci_job_definitions.

The JSON structure for the input definitions should look like this:

{
  "input_name_1": {
    "type": "string",
    "default": "default value"
  },
  "input_name_2": {
    "type": "array",
    "default": []
  }
}

Incorporate the comments left on Draft: Spike: Implement CI job inputs keyword (!202106 - closed) into the changes made for this issue.