Create consolidated group namespace filters table and model

As part of &12339, we need to add tables and models for namespace filters for group level external audit event destinations.

1. Already existing files for namespace filters for http destinations can be found at https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/models/audit_events/streaming/http/namespace_filter.rb, https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/models/audit_events/streaming/http/instance/namespace_filter.rb and https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/models/concerns/audit_events/streaming/http/namespace_filterable.rb.

2. Table schema would be:

create_table :audit_events_streaming_group_namespace_filters do |t|
t.timestamps_with_timezone null: false
t.references :external_streaming_destination,
null: false,
foreign_key: { to_table: 'audit_events_group_external_streaming_destinations', on_delete: :cascade }
t.references :namespace,
null: false,
foreign_key: { on_delete: :cascade }
t.index [:external_streaming_destination_id, :namespace_id], unique: true, name: UNIQ_INDEX_NAME
end

3. Please create a model named namespace_filter in https://gitlab.com/gitlab-org/gitlab/-/tree/master/ee/app/models/audit_events/group directory.
4. Create a concern namespace_filterable.rb for having common code for model in this issue as well as for instance #436606 (closed).
5. For cells, sharding key would be namespace_id.