[VC-43403] Refactor the CyberArk servicediscovery client to take an HTTP client

[wallrj-cyberark]

• Removed WithCustomEndpoint and WithHTTPClient options from the servicediscovery.Client.
• Updated New function to directly use http.Client and fetch the base URL from the ARK_DISCOVERY_API environment variable.
• Replaced ProdDiscoveryEndpoint constant with ProdDiscoveryAPIBaseURL for clarity.
• Refactored tests to use testing.T cleanup and environment variable setup for mock servers.
• Simplified test setup by removing redundant mock server initialization steps.

Follow up PRs

• [VC-43403] Refactor the CyberArk dataupload client to take an HTTP client #700
• [VC-43403] CyberArk: enhance service discovery to return multiple APIs #701
• [VC-43403] CyberArk(agent): add support for MachineHub output mode #696

Testing

Example of the extra logging from the tests, which helps diagnose test failures and helps to understand what API requests the client is making:

$ go test ./pkg/internal/cyberark/servicediscovery/... -v -args -testing.v 6
=== RUN   Test_DiscoverIdentityAPIURL
=== RUN   Test_DiscoverIdentityAPIURL/successful_request
    mock.go:69: GET /services/subdomain/venafi-test
    round_trippers.go:632: I0827 17:33:38.601555] Response verb="GET" url="https://127.0.0.1:36789/services/subdomain/venafi-test" status="200 OK" milliseconds=2
=== RUN   Test_DiscoverIdentityAPIURL/subdomain_not_found
    mock.go:69: GET /services/subdomain/something-random
    round_trippers.go:632: I0827 17:33:38.605067] Response verb="GET" url="https://127.0.0.1:35145/services/subdomain/something-random" status="404 Not Found" milliseconds=3
=== RUN   Test_DiscoverIdentityAPIURL/no_identity_service_in_response
    mock.go:69: GET /services/subdomain/no-identity
    round_trippers.go:632: I0827 17:33:38.607924] Response verb="GET" url="https://127.0.0.1:40485/services/subdomain/no-identity" status="200 OK" milliseconds=1
=== RUN   Test_DiscoverIdentityAPIURL/unexpected_HTTP_response
    mock.go:69: GET /services/subdomain/bad-request
    round_trippers.go:632: I0827 17:33:38.610534] Response verb="GET" url="https://127.0.0.1:36017/services/subdomain/bad-request" status="400 Bad Request" milliseconds=2
=== RUN   Test_DiscoverIdentityAPIURL/response_JSON_too_long
    mock.go:69: GET /services/subdomain/json-too-long
    round_trippers.go:632: I0827 17:33:38.696813] Response verb="GET" url="https://127.0.0.1:36707/services/subdomain/json-too-long" status="200 OK" milliseconds=85
=== RUN   Test_DiscoverIdentityAPIURL/response_JSON_invalid
    mock.go:69: GET /services/subdomain/json-invalid
    round_trippers.go:632: I0827 17:33:38.708833] Response verb="GET" url="https://127.0.0.1:32793/services/subdomain/json-invalid" status="200 OK" milliseconds=2
--- PASS: Test_DiscoverIdentityAPIURL (0.11s)
    --- PASS: Test_DiscoverIdentityAPIURL/successful_request (0.00s)
    --- PASS: Test_DiscoverIdentityAPIURL/subdomain_not_found (0.00s)
    --- PASS: Test_DiscoverIdentityAPIURL/no_identity_service_in_response (0.00s)
    --- PASS: Test_DiscoverIdentityAPIURL/unexpected_HTTP_response (0.00s)
    --- PASS: Test_DiscoverIdentityAPIURL/response_JSON_too_long (0.09s)
    --- PASS: Test_DiscoverIdentityAPIURL/response_JSON_invalid (0.00s)
PASS
ok      github.com/jetstack/preflight/pkg/internal/cyberark/servicediscovery    0.118s