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.go

@@ -428,6 +428,7 @@ func (b *Builder) Build() (*probodconfig.FullConfig, error) {
 		"CLICKUP",
 		"MONDAY",
 		"DATADOG",
+		"ZENDESK",
 	} {
 		clientID := b.getEnv("CONNECTOR_" + provider + "_CLIENT_ID")
 		if clientID == "" {
@@ -520,6 +521,7 @@ func (b *Builder) validateRequired() error {
 		{"CONNECTOR_CLICKUP", []string{"CLIENT_SECRET"}},
 		{"CONNECTOR_MONDAY", []string{"CLIENT_SECRET"}},
 		{"CONNECTOR_DATADOG", []string{"CLIENT_SECRET"}},
+		{"CONNECTOR_ZENDESK", []string{"CLIENT_SECRET"}},
 		{"CONNECTOR_VERCEL", []string{"CLIENT_SECRET", "INTEGRATION_SLUG"}},
 	}