Add verification header for streamed events

What does this MR do and why?

• Adds ability for audit stream consumers to verify origin of streamed events.
• Adds new database column verification_token to ExternalAuditEventDestinations which is generated automatically by postgresql. The migration also adds tokens to existing destinations too.
• Adds a new HTTP header to the streamed event which contains the verification token.
• Adds GraphQL read-only support for the new field.

Database review

Migrate

== 20220105082217 AddVerificationTokenToExternalAeDestinations: migrating =====
-- add_column(:audit_events_external_audit_event_destinations, :verification_token, :text)
   -> 0.0050s
== 20220105082217 AddVerificationTokenToExternalAeDestinations: migrated (0.0051s)

== 20220119094023 AddUniqueIndexToAedVerificationToken: migrating =============
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:audit_events_external_audit_event_destinations, :verification_token, {:unique=>true, :name=>"index_audit_events_external_audit_on_verification_token", :algorithm=>:concurrently})
   -> 0.0051s
-- execute("SET statement_timeout TO 0")
   -> 0.0010s
-- add_index(:audit_events_external_audit_event_destinations, :verification_token, {:unique=>true, :name=>"index_audit_events_external_audit_on_verification_token", :algorithm=>:concurrently})
   -> 0.0096s
-- execute("RESET statement_timeout")
   -> 0.0007s
== 20220119094023 AddUniqueIndexToAedVerificationToken: migrated (0.0274s) ====

== 20220119094503 PopulateAuditEventStreamingVerificationToken: migrating =====
== 20220119094503 PopulateAuditEventStreamingVerificationToken: migrated (0.3187s)

Rollback

== 20220119094503 PopulateAuditEventStreamingVerificationToken: reverting =====
== 20220119094503 PopulateAuditEventStreamingVerificationToken: reverted (0.0000s)

== 20220119094023 AddUniqueIndexToAedVerificationToken: reverting =============
-- transaction_open?()
   -> 0.0001s
-- indexes(:audit_events_external_audit_event_destinations)
   -> 0.0110s
-- execute("SET statement_timeout TO 0")
   -> 0.0066s
-- remove_index(:audit_events_external_audit_event_destinations, {:algorithm=>:concurrently, :name=>"index_audit_events_external_audit_on_verification_token"})
   -> 0.0185s
-- execute("RESET statement_timeout")
   -> 0.0062s
== 20220119094023 AddUniqueIndexToAedVerificationToken: reverted (0.0517s) ====

== 20220117082611 AddTextLimitToExadVerificationTokens: reverting =============
-- transaction_open?()
   -> 0.0000s
-- transaction_open?()
   -> 0.0000s
-- execute("ALTER TABLE audit_events_external_audit_event_destinations\nDROP CONSTRAINT IF EXISTS check_8ec80a7d06\n")
   -> 0.0043s
== 20220117082611 AddTextLimitToExadVerificationTokens: reverted (0.0682s) ====

Screenshots or screen recordings

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How to set up and validate locally

• Create some external audit event streaming destinations on a group. (Docs) - (use something like RequestBin as a desintation)
• Make sure the migration in this MR has been run locally.
  • Verify that the verification_token column has been created and populated (with randomly generated keys) for all existing desintations.
• Trigger the creation of audit events within a group (create projects, add members, etc.)
• Look at the streamed audit events, and ensure that they contain the X-Gitlab-Event-Streaming-Token header with the appropriate value.

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 #345424 (closed)