Set audit event HTTP destination

What does this MR do and why?

Adds the first part of the ability to stream audit events to an external service. This MR adds a new table that will store the HTTP endpoints to stream audit events to along with GraphQL mutations to add/update/destroy those destinations.

• Adds a new model ExternalAuditEventDestination and database table.
• Adds new GraphQL edge: Namespace -> ExternalAuditEventDestinations.
• Add mutations to create/update/destroy ExternalAuditEventDestination objects.
• Places all work behind default-off feature flag.
• Scopes work to GitLab Ultimate.
• Documentation - Moving this to the next stage as there's no functionality to document yet.

Screenshots or screen recordings

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Database Review

Migrate Down

== 20210819185500 CreateExternalAuditEventDestinationsTable: reverting ========
-- drop_table(:audit_events_external_audit_event_destinations)
   -> 0.0052s
== 20210819185500 CreateExternalAuditEventDestinationsTable: reverted (0.0255s)

Migrate Up

== 20210819185500 CreateExternalAuditEventDestinationsTable: migrating ========
-- create_table(:audit_events_external_audit_event_destinations, {})
-- quote_column_name(:destination_url)
   -> 0.0000s
   -> 0.0095s
== 20210819185500 CreateExternalAuditEventDestinationsTable: migrated (0.0096s)

How to set up and validate locally

1. Enable the feature. Disabled, none of the following steps should function.

   Feature.enable(:ff_external_audit_events_namespace)

⚠ Note: These changes should only work for group owners.

Create a destination

1. Execute the externalAuditEventDestinationCreate mutation. Here's an example use of the mutation:

mutation {
  externalAuditEventDestinationCreate(input: { destinationUrl: "htfdftps://test.io", groupPath: "{GROUP PATH THAT current_user IS AN OWNER OF}" } ) {
    errors
    externalAuditEventDestination {
      destinationUrl
      group {
        name
      }
    }
  }
}

List all destinations

query {
  group(fullPath: "example-group-1") {
    id
    externalAuditEventDestinations {
      nodes {
        destinationUrl
        id
      }
    }
  }
}

Update a destination

1. Execute the externalAuditEventDestinationUpdate mutation. Here's an example use of the mutation:

mutation {
  externalAuditEventDestinationUpdate(input: { id: "gid://gitlab/AuditEvents::ExternalAuditEventDestination/1", destinationUrl: "https://fdsfdsfsd.com"} ) {
    errors
    externalAuditEventDestination {
      id
      destinationUrl
    }
  }
}

Destroy a destination

1. Execute the externalAuditEventDestinationDestroy mutation. Here's an example use of the mutation:

mutation { externalAuditEventDestinationDestroy(input: { id: "gid://gitlab/AuditEvents::ExternalAuditEventDestination/1" } ) { errors } }

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 #332746 (closed)