Add SendGrid access review driver

Implement a SendGrid access-review driver that fetches teammates
from the SendGrid API and maps them into AccountRecord values.

Register SendGrid as a connector provider, expose it through the
connector provider enum, and add a migration that extends the
connector_provider type with SENDGRID.

Cover the new driver with a VCR-backed fixture test and helper
tests for role and response-shape handling to keep parsing robust.

Signed-off-by: Cursor Agent <cursoragent@cursor.com>

Co-authored-by: Bryan FRIMIN <bryan@frimin.fr>

pkg/connector/provider/builtin.go

@@ -49,6 +49,7 @@ func NewBuiltinRegistry() *Registry {
 		posthogRegistration(),
 		pagerdutyRegistration(),
 		resendRegistration(),
+		sendgridRegistration(),
 		sentryRegistration(),
 		slackRegistration(),
 		supabaseRegistration(),