test(mtls): add e2e tests for mTLS client cert identity forwarding

Tests verify: mTLS proxy works with valid client cert, mTLS rejects
connections without client cert, HTTP auth plugin receives client_cn,
client_san, client_cert_fingerprint via PeerCert context propagation.

tests/e2e/testdata/mtls/server.yaml

@@ -0,0 +1,17 @@
+services:
+- name: https-mtls
+  addr: :8443
+  handler:
+    type: http
+    auther: auther-0
+  listener:
+    type: tls
+    tls:
+      certFile: /certs/server.pem
+      keyFile: /certs/server-key.pem
+      caFile: /certs/ca.pem
+authers:
+- name: auther-0
+  plugin:
+    type: http
+    addr: http://{{.ServerAddr}}:9000/auth