Validate ci_job_definitions.config with JSON schema

Add JSON schema validation for config column (reuse existing schema for id_tokens and secrets) - We should be able to reuse a lot of the schema introduced in Control what data is persisted in `p_ci_builds_... (#547426 - closed).

{
  "options": {...},   // json schema exists for ci_builds_metadata.config_options
  "yaml_variables": {...},
  "id_tokens": {...}, // json schema exists for ci_builds_metadata.id_tokens
  "secrets": {...},   // json schema exists for ci_builds_metadata.secrets
  "interruptible": true
}