feat(hybrid-cloud): Add webhook identifier for getsentry requests (#55418)

This identifier will act as a catch all for webhooks consumed in the
getsentry codebase. This will avoid having to make a sentry change for
adding new webhook parsers in getsentry.

The alternative is to instead create identifiers for `salesforce`,
`github_marketplace` and `stripe` which do not occur in this codebase.

src/sentry/models/outbox.py

@@ -132,6 +132,7 @@ class WebhookProviderIdentifier(IntEnum):
     GITHUB_ENTERPRISE = 8
     BITBUCKET_SERVER = 9
     LEGACY_PLUGIN = 10
+    GETSENTRY = 11
 
 
 def _ensure_not_null(k: str, v: Any) -> Any: