Added update api for instance level audit streaming

What does this MR do and why?

Adding GraphQL API for updating newly added consolidated instance level external audit event streaming destinations.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

1. Create instance level destination by following steps mentioned in !148383 (merged).
2. Then you can update the destination by running following mutation

mutation instanceAuditEventStreamingDestinationsUpdate{
  instanceAuditEventStreamingDestinationsUpdate(input: {
    id:"gid://gitlab/AuditEvents::Instance::ExternalStreamingDestination/3",
    config: {
      bucket: "some-bucket",
      awsRegion: "us-east-1",
      accessKeyID: "someRandomKey"
    },
    category: "aws",
    secretToken: "aws-random-token"
  } ) {
    errors
    externalAuditEventDestination {
      id
      name
      config
      category
    }
  }
}

Related to #436615 (closed)