Allow OR params in GraphQL issue resolvers

What does this MR do and why?

Support OR params in GraphQL issues and issue boards. Author and assignee filters are already supported at the finder level so this just adds the param to GraphQL.

Enable the or_issuable_queries feature flag to test this feature.

Sample query

query {
  project(fullPath: "flightjs/flight") {
    issues(or: { authorUsernames: ["user1", "user2"] }) {
      nodes {
        id
        title
        author {
          username
        }
        assignees {
          nodes {
            username
          }
        }
      }
    }
  }
}

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #296031 (closed)