Add ability to search iterations by cadence name

In &5077 (comment 613272176), @gweaver identified the need to "figure out how to allow me to search and select an iteration within a cadence by cadence name".

Currently, we can search for iterations by iteration title:

// only searches by iteration title
query{
  project(fullPath: "gitlab-org/gitlab") {
    iterations(title: "gitlab") {
      nodes {
        id
        title
      }
    }
  }
}

We need to be able to search for iterations by cadence name too.

Proposal

We can currently search epics by epic title only:

// searches epics by title only
{
  group(fullPath: "gitlab-org") {
    epics(search: "pajamas", in: [TITLE]) {
      nodes {
        id
        title
      }
    }
  }
}

We could similarly add the ability to search iterations by cadence name with the following query:

// proposal to search iterations by cadence name — return iterations whose cadence name includes the search term
query{
  project(fullPath: "gitlab-org/gitlab") {
    iterations(search: "my cadence name", in: [CADENCE_TITLE]) {
      nodes {
        id
        title
      }
    }
  }
}

// proposal to search iterations by cadence name — return iterations whose title or cadence name includes the search term
query{
  project(fullPath: "gitlab-org/gitlab") {
    iterations(search: "my cadence name", in: [TITLE, CADENCE_TITLE]) {
      nodes {
        id
        title
      }
    }
  }
}

This would involve:

1. Creating a new parameter search for doing a broad search
2. Creating a new parameter in to specify which fields to search across, with TITLE and CADENCE_TITLE being enum options
3. Deprecating the parameter title. Its new equivalent parameters would be search: "my iteration title", in: [TITLE]

Acceptance criteria

• The user can search iterations by cadence name via GraphQL