Only apply rewrites to mirror endpoints

[brandond]

• Makes behavior consistent with recent releases of K3s and RKE2
• Fixes Rewrite configuration is applied to the registry fallback #25

Test confirms that the rewrites are applied when pulling from a mirror endpoint, but not when pulling from the default endpoint:

=== RUN   TestImage/Pull_busybox_without_rewrite_on_default_endpoint
time="2024-10-01T21:40:50Z" level=info msg="generated self-signed CA certificate CN=dynamiclistener-ca@1727818850,O=dynamiclistener-org: notBefore=2024-10-01 21:40:50.005892519 +0000 UTC notAfter=2034-09-29 21:40:50.005892519 +0000 UTC"
time="2024-10-01T21:40:50Z" level=info msg="certificate CN=127-0-0-1.sslip.io,O=TestImage/Pull_busybox_without_rewrite_on_default_endpoint signed by CN=dynamiclistener-ca@1727818850,O=dynamiclistener-org: notBefore=2024-10-01 21:40:50 +0000 UTC notAfter=2025-10-01 21:40:50 +0000 UTC"
    endpoint_test.go:50: INFO: TestImage/Pull_busybox_without_rewrite_on_default_endpoint registry 127-0-0-1.sslip.io at https://127-0-0-1.sslip.io, auth 127-0-0-1.sslip.io at https://127-0-0-1.sslip.io, scheme "Basic"
=== RUN   TestImage/Pull_busybox_without_rewrite_on_default_endpoint/library/busybox:latest
time="2024-10-01T21:40:50Z" level=debug msg="Trying endpoint https://127-0-0-1.sslip.io:443/v2"
time="2024-10-01T21:40:50Z" level=debug msg="Registry endpoint URL modified: https://127-0-0-1.sslip.io/v2/ => https://127-0-0-1.sslip.io:443/v2/?ns=127-0-0-1.sslip.io"
time="2024-10-01T21:40:50Z" level=debug msg="Registry endpoint URL modified: https://127-0-0-1.sslip.io/v2/bogus-image-prefix/busybox/manifests/latest => https://127-0-0-1.sslip.io:443/v2/bogus-image-prefix/busybox/manifests/latest?ns=127-0-0-1.sslip.io"
time="2024-10-01T21:40:50Z" level=warning msg="Failed to get image from endpoint: GET https://127-0-0-1.sslip.io:443/v2/bogus-image-prefix/busybox/manifests/latest?ns=REDACTED: unexpected status code 404 Not Found"
time="2024-10-01T21:40:50Z" level=debug msg="Trying endpoint https://127-0-0-1.sslip.io/v2"
    endpoint_test.go:103: OK: 127-0-0-1.sslip.io/library/busybox:latest