Migrations and models for consolidated streaming destination for groups

What does this MR do and why?

Currently we have multiple tables for storing audit events external streaming destinations for top-level groups. Different tables are for storing configs of different types of destinations, currently custom http, aws and GCP. This changes intends to unify all of those tables into a single one for ease and increasing speed of integrations. This part of proposal made in &12339.

1. As part of this MR, we are adding table and model for storing configs of different types of audit event streaming destinations for top-level group.
2. We will be adding the validations of different types of configs in later MRs.
3. I have added a concern and shared examples for `externally_streamable.rb` by keeping in mind that we are soon going to do the same for instance level audit event streaming destinations too. So, for now it may seem redundant but it is going to be used.

Migrations

Up

bundle exec rake db:migrate:up:main VERSION=20240112124030

main: == [advisory_lock_connection] object_id: 115880, pg_backend_pid: 69674
main: == 20240112124030 CreateAuditEventsGroupExternalStreamingDestinations: migrating
main: -- create_table(:audit_events_group_external_streaming_destinations)
main: -- quote_column_name(:name)
main:    -> 0.0000s
main: -- quote_column_name(:type)
main:    -> 0.0000s
main:    -> 0.0090s
main: == 20240112124030 CreateAuditEventsGroupExternalStreamingDestinations: migrated (0.0343s)main: == [advisory_lock_connection] object_id: 115880, pg_backend_pid: 69674

Down

bundle exec rake db:migrate:down:main VERSION=20240112124030

main: == [advisory_lock_connection] object_id: 117080, pg_backend_pid: 69108
main: == 20240112124030 CreateAuditEventsGroupExternalStreamingDestinations: reverting
main: -- drop_table(:audit_events_group_external_streaming_destinations)
main:    -> 0.0096s
main: == 20240112124030 CreateAuditEventsGroupExternalStreamingDestinations: reverted (0.0218s)main: == [advisory_lock_connection] object_id: 117080, pg_backend_pid: 69108

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Before	After

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Related to #436601 (closed)