feat(projectconfig): Option for client-based transaction name handling (#37104)

Prepare option for #37102. See description of that PR.

src/sentry/options/defaults.py

@@ -408,6 +408,11 @@ register("relay.drop-transaction-metrics", default=[])
 #       given fraction of orgs even if the corresponding feature flag is disabled.
 register("relay.transaction-metrics-org-sample-rate", default=0.0)
 
+# Sample rate for opting in orgs into the new transaction name handling.
+# old behavior: Treat transactions from old SDKs as high-cardinality.
+# new behavior: Treat transactions from old SDKs as low-cardinality, except for browser JS.
+register("relay.transaction-names-client-based", default=0.0)
+
 # Write new kafka headers in eventstream
 register("eventstream:kafka-headers", default=True)