refactor(retrofit): remove all com.squareup.okhttp dependencies

[kirangodishala]

All references to com.squareup.okhttp will be removed with this PR.

ApiExtensionController needed minor refactoring to fix the following error caused due to kotlin-java conflicts:

class java.lang.Object cannot be cast to class java.lang.String (java.lang.Object and java.lang.String are in module java.base of loader 'bootstrap')
  java.lang.ClassCastException: class java.lang.Object cannot be cast to class java.lang.String (java.lang.Object and java.lang.String are in module java.base of loader 'bootstrap')
  at com.netflix.spinnaker.gate.controllers.ApiExtensionController.any(ApiExtensionController.kt:80)
  at com.netflix.spinnaker.gate.controllers.ApiExtensionControllerTest$tests$1$3$2.invoke(ApiExtensionControllerTest.kt:61)
  at com.netflix.spinnaker.gate.controllers.ApiExtensionControllerTest$tests$1$3$2.invoke(ApiExtensionControllerTest.kt:59)

Refer spinnaker/kork#1210 for kork PR which completes the removal of okhttp2 dependencies.