chore(staff): Create staff email allowlist option (#67742)

In preparation to replace with `auth:enterprise-staff-cookie` feature
flag with this option so I can easily modify who has access to staff
mode with the options automator, rather than having to pass CI with the
feature flag handler

src/sentry/options/defaults.py

@@ -264,6 +264,14 @@ register(
     flags=FLAG_ALLOW_EMPTY | FLAG_PRIORITIZE_DISK | FLAG_REQUIRED,
 )
 
+# Staff
+register(
+    "staff.user-email-allowlist",
+    type=Sequence,
+    default=[],
+    flags=FLAG_ALLOW_EMPTY | FLAG_AUTOMATOR_MODIFIABLE,
+)
+
 # API
 # Killswitch for apis to work with id or slug as path parameters
 register(