Add Zendesk access-review connector

Zendesk is a multi-tenant OAuth connector keyed by the customer
subdomain. The customer enters it at connect time; it rides the signed
state to the callback, is re-validated, and is stored on the connector
settings to build the API host.

List staff (agents and admins) via GET /api/v2/users.json with cursor
pagination, mapping role, active/suspended, and 2FA status; end-users
are excluded. The subdomain is validated as a single DNS label at every
trust boundary to close the SSRF vector, and the data client keeps the
SSRF-protected transport.

Zendesk OAuth across customer subdomains requires a Zendesk-approved
global OAuth client; the connector goes live once those credentials are
supplied via bootstrap.

Signed-off-by: Aurélien Sibiril <81782+aureliensibiril@users.noreply.github.com>

pkg/server/api/console/v1/graphql/connector.graphql

@@ -61,6 +61,7 @@ enum ConnectorProvider
     CURSOR @goEnum(value: "go.probo.inc/probo/pkg/coredata.ConnectorProviderCursor")
     DATADOG @goEnum(value: "go.probo.inc/probo/pkg/coredata.ConnectorProviderDatadog")
     OKTA @goEnum(value: "go.probo.inc/probo/pkg/coredata.ConnectorProviderOkta")
+    ZENDESK @goEnum(value: "go.probo.inc/probo/pkg/coredata.ConnectorProviderZendesk")
 }
 
 type ConnectorProviderInfo {