[Feature Request] /search api endpoint should include job metadata

It would be nice to be able to search for all jobs that have a particular metadata in the job definition.

For example, at my company we add the following metadata to jobs.

```hcl
meta {
project_url = "<%= ENV['CI_PROJECT_URL'] %>"
branch = "<%= ENV['CI_COMMIT_REF_NAME'] %>"
ephemeral = "<%= ENV.fetch('EPHEMERAL', false) %>"
}
```

A ci process converts this to

```hcl
meta {
project_url = "gitlab.example.com"
branch = "feature_branch1"
ephemeral = "true"
}
```

We would like to use the `/search` api to search for all jobs that are more than 24 hours old, and have `ephemeral = true` set in them.

Start by tracing the `/search` API endpoint and how job-definition metadata is represented and queried. Define how searches for metadata such as `ephemeral = true` should combine with the requested age filter, then verify that the endpoint returns only matching jobs.

Written by the indexing model from the issue text.