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/bootstrap/builder_test.go

@@ -522,6 +522,7 @@ func TestBuilder_Build_AccessReviewConnectors(t *testing.T) {
 	providers := []string{
 		"GITLAB", "BITBUCKET", "HEROKU", "PAGERDUTY",
 		"ASANA", "NETLIFY", "CLICKUP", "MONDAY", "DATADOG",
+		"ZENDESK",
 	}
 
 	env := requiredEnv()