Add auditEventsStreamingHeadersDestroy Mutation

What does this MR do and why?

• Adds a new graphQL mutation auditEventsStreamingHeadersDestroy to destroy a custom header on a streaming destination.

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How to set up and validate locally

• Enable the streaming_audit_event_headers feature flag.
• Create an Ultimate group.
• Create a streaming destination. (Group -> Security & Compliance -> Audit Events -> Streams)
• Using GraphQL, create a header.

mutation {
  auditEventsStreamingHeadersCreate(input: { destinationId: "gid://gitlab/AuditEvents::ExternalAuditEventDestination/ID GOES HERE", key: "X-Foo", value: "bar" }) {
    errors
    clientMutationId
    header {
      id
      value
      key
    }
  }
}

• Using the returned ID value, delete the same header.

mutation {
  auditEventsStreamingHeadersDestroy(input: { headerId: "gid://gitlab/AuditEvents::Streaming::Header/7" }) {
    errors
    clientMutationId
  }
}

• Ensure that the header has been deleted from the database.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #361217 (closed)

doc/administration/audit_event_streaming.md

@@ -166,6 +166,18 @@ mutation {
 }
 ```
 
+### Delete with the API
+
+Group owners can remove a HTTP header using the GraphQL `auditEventsStreamingHeadersDestroy` mutation.
+
+```graphql
+mutation {
+  auditEventsStreamingHeadersDestroy(input: { headerId: "gid://gitlab/AuditEvents::ExternalAuditEventDestination/24601" }) {
+    errors
+  }
+}
+```
+
 The header is created if the returned `errors` object is empty.
 
 ## Verify event authenticity