Adding update instance headers API

What does this MR do and why?

Added GraphQL API for updating streaming headers for instance level external audit event destinations.

GraphQL API

mutation {
  auditEventsStreamingInstanceHeadersUpdate(input: { headerId: "gid://gitlab/AuditEvents::Streaming::InstanceHeader/3", key: "new-key", value: "new-value" }) {
    errors
    header {
      id
      key
      value
    }
  }
}

Query plan

Query - Click to expand

UPDATE 
  "instance_audit_events_streaming_headers" 
SET 
  "updated_at" = '2023-06-12 12:58:24.503241', 
  "key" = 'new-key', 
  "value" = 'new-value' 
WHERE 
  "instance_audit_events_streaming_headers"."id" = 40

https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/19381/commands/64007

Note: Since this is an instance level API, there are no records on production.

How to set up and validate locally

1. Enable the feature flag from rails console by running Feature.enable(:ff_external_audit_events).
2. Open graphql-explorer console in browser by opening url http://127.0.0.1:3000/-/graphql-explorer.
3. Run following mutation in the graphql explorer and note down the id, which would be something like gid://gitlab/AuditEvents::InstanceExternalAuditEventDestination/<id> from output.

mutation {
  instanceExternalAuditEventDestinationCreate(input: { destinationUrl: "https://www.example.com"}) {
    errors
    instanceExternalAuditEventDestination {
      destinationUrl
      id
    }
  }
}

4. Run the following graphql command in explorer for creating a header created for destination created in step 3, note down the id of header:

mutation {
  auditEventsStreamingInstanceHeadersCreate(input: 
    { 
      key: "header1",
      value: "header1value",
      destinationId: "gid://gitlab/AuditEvents::InstanceExternalAuditEventDestination/<id>"
    }) {
    errors
    header {
      id
      key
      value
    }
  }
}

5. Run following mutation for updating the header, it should return with a successful response:

mutation {
  auditEventsStreamingInstanceHeadersUpdate(input: { headerId: "gid://gitlab/AuditEvents::Streaming::InstanceHeader/<id>", key: "new-key", value: "new-value" }) {
    errors
    header {
      id
      key
      value
    }
  }
}

6. You can also check the updated value in rails console by running AuditEvents::Streaming::InstanceHeader.last.key or simply AuditEvents::Streaming::InstanceHeader.last.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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

Related to #404560 (closed)