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/accessreview/drivers/testdata/zendesk.yaml

@@ -0,0 +1,82 @@
+---
+version: 2
+interactions:
+    - id: 0
+      request:
+        proto: HTTP/1.1
+        proto_major: 1
+        proto_minor: 1
+        content_length: 0
+        transfer_encoding: []
+        trailer: {}
+        host: acme.zendesk.com
+        remote_addr: ""
+        request_uri: ""
+        body: ""
+        form:
+            page[size]:
+                - "100"
+            role[]:
+                - agent
+                - admin
+        headers:
+            Accept:
+                - application/json
+        url: https://acme.zendesk.com/api/v2/users.json?page%5Bsize%5D=100&role%5B%5D=agent&role%5B%5D=admin
+        method: GET
+      response:
+        proto: HTTP/2.0
+        proto_major: 2
+        proto_minor: 0
+        transfer_encoding: []
+        trailer: {}
+        content_length: -1
+        uncompressed: true
+        body: |
+            {
+              "users": [
+                {
+                  "id": 12345,
+                  "email": "alice@example.com",
+                  "name": "Alice Example",
+                  "role": "admin",
+                  "suspended": false,
+                  "active": true,
+                  "two_factor_auth_enabled": true,
+                  "last_login_at": "2025-06-01T10:00:00Z",
+                  "created_at": "2025-01-02T03:04:05Z"
+                },
+                {
+                  "id": 67890,
+                  "email": "bob@example.com",
+                  "name": "Bob Example",
+                  "role": "agent",
+                  "suspended": false,
+                  "active": true,
+                  "two_factor_auth_enabled": false,
+                  "last_login_at": null,
+                  "created_at": "2025-02-01T00:00:00Z"
+                },
+                {
+                  "id": 99999,
+                  "email": "carol@example.com",
+                  "name": "Carol Customer",
+                  "role": "end-user",
+                  "suspended": false,
+                  "active": true,
+                  "two_factor_auth_enabled": false,
+                  "last_login_at": null,
+                  "created_at": "2025-03-01T00:00:00Z"
+                }
+              ],
+              "meta": {
+                "has_more": false,
+                "after_cursor": null
+              }
+            }
+        headers:
+            Content-Type:
+                - application/json
+        status: 200 OK
+        code: 200
+        duration: 1ms