Add legacy_destination_ref column and mappable concern

What does this MR do and why?

While we are in the in-between stage of migrating from the old audit event destination models and the new ones, we need a link between the different kinds of models, to map to our new ones.

This MR adds a legacy_destination_ref to both new audit event streaming destination models, as an integer. The integer will represent an ID (PK) from either:

• HTTP: AuditEvents::InstanceExternalAuditEventDestination, AuditEvents::ExternalAuditEventDestination
• AWS: AuditEvents::Instance::AmazonS3Configuration, AuditEvents::AmazonS3Configuration
• GCP: AuditEvents::Instance::GoogleCloudLoggingConfiguration, AuditEvents:GoogleCloudLoggingConfiguration

In the two new models: AuditEvents::Instance::ExternalStreamingDestination,AuditEvents::Group::ExternalStreamingDestination

I've added a new LegacyDestinationMappable concern that will map the category of the current model and the id back to the correct old model: category (http/aws/gcp) + group/instance is enough to distinguish an answer of those 6 models.

In the future when we deprecate the old models and remove old code, we'll just drop the columns and remove the concern.

This migration DOES NOT include a backfill of old existing records, as that will be done in another issue/MR. There will be future work for when these old models are created, that we also create the new models, with this legacy_destination_ref populated.

References

• #503244 (closed)

How to set up and validate locally

• Checkout the branch and migrate (to add the two new columns)
• Enter rails console
• Create a destination (old) like legacy_http = AuditEvents::InstanceExternalAuditEventDestination.create!(name: "Test", destination_url: "https://gitlab.com")
• Create a new destination, with a legacy_destination_ref of the one we just created: new_http = AuditEvents::Instance::ExternalStreamingDestination.create!(name: "Test", category: :http, legacy_destination_ref: legacy_http.id, config: { url: "https://gitlab.com" })
• Check new_http.legacy_destination and new_http.legacy_destination_ref (eq new_http.legacy_destionation.id)
• Do this for group/instance + aws/http/gcp until satisfied

Example Console Output:

legacy_http = AuditEvents::InstanceExternalAuditEventDestination.create!(name: "Test", destination_url: "https://gitlab.com")

=> #<AuditEvents::InstanceExternalAuditEventDestination:0x0000000156cb4740
 id: 2,
 created_at: Mon, 11 Nov 2024 20:44:10.670572000 UTC +00:00,
 updated_at: Mon, 11 Nov 2024 20:44:10.670572000 UTC +00:00,
 destination_url: "https://gitlab.com",
 encrypted_verification_token: "[FILTERED]",
 encrypted_verification_token_iv: "\x89\x18\xEEj(\xF1\x92\xF7J\xA1\x83\xF3",
 name: "Test",
 verification_token: nil

new_http = AuditEvents::Instance::ExternalStreamingDestination.create!(name: "Test", category: :http, legacy_destination_ref: legacy_http.id, config: { url: "https://example.com" })

=> #<AuditEvents::Instance::ExternalStreamingDestination:0x000000016b810eb8
 id: 2,
 created_at: Mon, 11 Nov 2024 20:44:20.570798000 UTC +00:00,
 updated_at: Mon, 11 Nov 2024 20:44:20.570798000 UTC +00:00,
 category: "http",
 name: "Test",
 config: {"url"=>"https://example.com"},
 encrypted_secret_token: "[FILTERED]",
 encrypted_secret_token_iv: "[FILTERED]",
 legacy_destination_ref: 2,
 secret_token: nil>


> new_http.legacy_destination
=> #<AuditEvents::InstanceExternalAuditEventDestination:0x0000000156c1a280
 id: 2,
 created_at: Mon, 11 Nov 2024 20:44:10.670572000 UTC +00:00,
 updated_at: Mon, 11 Nov 2024 20:44:10.670572000 UTC +00:00,
 destination_url: "https://gitlab.com",
 encrypted_verification_token: "[FILTERED]",
 encrypted_verification_token_iv: "\x89\x18\xEEj(\xF1\x92\xF7J\xA1\x83\xF3",
 name: "Test",
 verification_token: nil>