Add Better Stack access review connector

Better Stack exposes team members and pending invitations through its
Uptime API. Wire it as an access-review connector so a Better Stack
team can be reviewed in access-review campaigns.

Better Stack has no third-party OAuth app for listing members (its
OAuth is an end-user MCP sign-in), so the connector authenticates with
a Bearer API token plus the team name that scopes the team-members
listing. The driver paginates /api/v2/team-members, maps roles and
invitation records into account records, and the source name is
resolved from the configured team.

This wires the full surface: the provider enum and migration, the
connector settings, the registry registration with the team-name extra
setting, the GraphQL input and resolver marshaling, the frontend field
mapping and connector logo, and cassette-backed driver tests.

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

pkg/accessreview/drivers/testdata/better_stack.yaml

@@ -0,0 +1,33 @@
+---
+version: 2
+interactions:
+    - id: 0
+      request:
+        proto: HTTP/1.1
+        proto_major: 1
+        proto_minor: 1
+        content_length: 0
+        host: betterstack.com
+        form:
+            page:
+                - "1"
+            team_name:
+                - acme
+        headers:
+            Accept:
+                - application/json
+        url: https://betterstack.com/api/v2/team-members?page=1&team_name=acme
+        method: GET
+      response:
+        proto: HTTP/2.0
+        proto_major: 2
+        proto_minor: 0
+        content_length: -1
+        uncompressed: true
+        body: '{"data":[{"id":"101","type":"team_member","attributes":{"email":"alice@example.com","first_name":"Alice","last_name":"Smith","created_at":"2026-06-09T13:24:53.712Z","role":"admin","mobile_app_platforms":[]}}],"pagination":{"first":"https://betterstack.com/api/v2/team-members?page=1&team_name=acme","last":"https://betterstack.com/api/v2/team-members?page=1&team_name=acme","prev":null,"next":null}}'
+        headers:
+            Content-Type:
+                - application/json; charset=utf-8
+        status: 200 OK
+        code: 200
+        duration: 100ms