Generate an additive JSON Patch instead of overwriting containers

[iamnoah]

Issue #, if available: 165

Description of changes:

The problem in #165 is that the code is serializing the k8s.io structs as is, with an 'add' operation that effectively replaces the Container in the PodSpec. This works until fields are added, like grpc, at which point this webhook has to be updated, or it will drop the new fields.

Instead of serializing the entire container struct, this uses jsonpatch to create a diff from the modifications, and generated a targeted patch that only adds and modifies the things it needs to. Unknown fields will not be dropped.

I've not updated the test cases as they are hardcoded to expect a specific patch (that will drop unknown fields.) TBH, that approach needs to be rethought.

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[iamnoah]

To be clear, the goal of this PR is to change the resulting patch. Here is an example diff from the unit tests:

--- want
+++ got
@@ -21,29 +21,26 @@
   },
   {
     "op": "add",
-    "path": "/spec/containers",
+    "path": "/spec/containers/0/volumeMounts",
     "value": [
       {
-        "name": "balajilovesoreos",
-        "image": "amazonlinux",
-        "env": [
+        "mountPath": "/var/run/secrets/eks.amazonaws.com/serviceaccount",
+        "name": "aws-iam-token",
+        "readOnly": true
+      }
+    ]
+  },
   {
+    "op": "add",
+    "path": "/spec/containers/0/env",
+    "value": [
+      {
             "name": "AWS_ROLE_ARN",
             "value": "arn:aws:iam::111122223333:role/s3-reader"
           },
           {
             "name": "AWS_WEB_IDENTITY_TOKEN_FILE",
             "value": "C:\\var\\run\\secrets\\eks.amazonaws.com\\serviceaccount\\token"
-          }
-        ],
-        "resources": {},
-        "volumeMounts": [
-          {
-            "name": "aws-iam-token",
-            "readOnly": true,
-            "mountPath": "/var/run/secrets/eks.amazonaws.com/serviceaccount"
-          }
-        ]
       }
     ]
   }

The new patch is more targeted but it adds the same values to env and mounts. The omission of the name and image are a good thing, because that means the rest of the container spec is unchanged.