List all streaming audit event headers for a destination

Overview

Once the ability to create a streaming event header is released, we need to allow a group owner to see those headers via the GraphQL API.

Proposal

Implement a query like this:

query {
  group(fullPath: "example-group") {
    id
    externalAuditEventDestinations {
      nodes {
        destinationUrl
        verificationToken
        id
        headers { 
          nodes {
            key
            value
          }
        }
      }
    }
  }
}