feat(api)!: remove methods 'customers.usage.create' and 'customers.us…

…age.update_by_external_id' (#30)

## Migration
The two methods `customers.usage.create` and `customers.usage.update_by_external_id` have been removed from the SDK and the API.
Please use 'events.backfills.create' instead.
More details for this endpoint can be found at https://docs.withorb.com/reference/create-backfill

.stats.yml

@@ -1,2 +1,2 @@
-configured_endpoints: 93
-openapi_spec_url: https://storage.googleapis.com/stainless-sdk-openapi-specs/orb-e620baef7c2e237e0f03f462c688ba7bb1215d6481139b56b0074b6022d0ce6e.yml
+configured_endpoints: 91
+openapi_spec_url: https://storage.googleapis.com/stainless-sdk-openapi-specs/orb-83ae433113181f3839cc63db01cc7c815de1fff597a1cbaf8ffda48ba98268c9.yml