Update kubernetai plugin to support transfers via the transfers plugin.

[davidmccormick]

The kubernetes plugin was updated to support a new transfer plugin for managing zone transfer requests:

see:-

coredns/coredns#2585 - plugin/etcd: support zone transfers #2585
coredns/coredns#3121 - plugin/transfer: New plugin for handling zone transfers #3121
coredns/coredns#3223 - plugin/transfer: Zone transfer plugin #3223

When this work was performed, the internal implementation of AXFR requests was removed from the kubernetes plugin and instead it supports the transfer.Transferer interface. kubernetai doesn't support the transfer.Transferer interface and so building CoreDNS v1.11.3 with this plugin results in AXFR requests failing to respond with the zones.

This PR adds a Transfer method to the Kubernetesai object so that it can broker AXFR requests to its embedded Kubernetes plugins.

In order to write tests, I have abstracted the Kubernetes plugin to a embeddedKubernetesPluginInterface. Which allows me to swap out the underlying Kubernetes plugin. This is neater than the podHandlerItf approach and allows for us to generate different responses from the embedded plugins needed to check the integration through kubernetes plugins.

This PR is also supported by integration tests in coredns/ci - coredns/ci#158

The kubernetai tests fail against master:

=== RUN   TestAXFR
=== RUN   TestAXFR/matches_stanza_1/test-4.svc.cluster.local._AXFR
    axfr_test.go:187: expected 1 query attempt, observed 0
    axfr_test.go:196: coredns log: .:53
        CoreDNS-1.11.3
        linux/arm64, go1.23.1, ce45312
        [INFO] 172.17.0.1:37727 - 13103 "AXFR IN test-4.svc.cluster.local. tcp 65 false 65535" - - 0 0.000046625s
=== RUN   TestAXFR/matches_stanza_2/test-5.svc.cluster.local._AXFR
    axfr_test.go:187: expected 1 query attempt, observed 0
    axfr_test.go:196: coredns log: .:53
        CoreDNS-1.11.3
        linux/arm64, go1.23.1, ce45312
        [INFO] 172.17.0.1:36319 - 63827 "AXFR IN test-5.svc.cluster.local. tcp 65 false 65535" - - 0 0.0000605s
=== RUN   TestAXFR/matches_first_stanza/cluster.local._AXFR
    axfr_test.go:187: expected 1 query attempt, observed 0
    axfr_test.go:196: coredns log: .:53
        CoreDNS-1.11.3
        linux/arm64, go1.23.1, ce45312
        [INFO] 172.17.0.1:57837 - 14074 "AXFR IN cluster.local. tcp 54 false 65535" - - 0 0.00003625s
--- FAIL: TestAXFR (9.17s)
    --- FAIL: TestAXFR/matches_stanza_1/test-4.svc.cluster.local._AXFR (3.09s)
    --- FAIL: TestAXFR/matches_stanza_2/test-5.svc.cluster.local._AXFR (3.11s)
    --- FAIL: TestAXFR/matches_first_stanza/cluster.local._AXFR (2.97s)
FAIL
FAIL	github.com/coredns/ci/test/kubernetai	24.938s
FAIL

Manual testing against this PR produces:

cd $GOPATH/src/github.com/coredns/ci && go mod tidy && go test -v ./test/kubernetai/... --run=TestAXFR                                                                                                                               
=== RUN   TestAXFR
=== RUN   TestAXFR/matches_stanza_1/test-4.svc.cluster.local._AXFR
=== RUN   TestAXFR/matches_stanza_2/test-5.svc.cluster.local._AXFR
=== RUN   TestAXFR/matches_first_stanza/cluster.local._AXFR
--- PASS: TestAXFR (8.59s)
    --- PASS: TestAXFR/matches_stanza_1/test-4.svc.cluster.local._AXFR (2.50s)
    --- PASS: TestAXFR/matches_stanza_2/test-5.svc.cluster.local._AXFR (3.01s)
    --- PASS: TestAXFR/matches_first_stanza/cluster.local._AXFR (3.08s)
PASS
ok      github.com/coredns/ci/test/kubernetai    9.297s 

[davidmccormick]

The test has passed because the AXFR test has yet to be merged - coredns/ci#158

[davidmccormick]

If this PR is merged before #66 we should see the existing kubernetai plugin fail these tests because it doesn't support the transfer plugin.