Add `app-change describe` command to allow the user to view diffs for deployed changes

[100mik]

Describe the problem/challenge you have As of today, kapp app-change list -a app_name lists the duration of changes deployed for an application and displays a summary of the operations performed. This feature could be extended to provide the user more insight by making the text diff for each change available.

Describe the solution you'd like The config maps used to track app-changes can be used to store text diffs as well. And these can be exposed to the user through an app-change describe command.

The end result might be similar to this:

❯ kapp app-change list -a test                                                                                                                                                                                             ─╯
Target cluster 'https://127.0.0.1:57406' (nodes: minikube)

App changes

Name               Started At            Finished At           Successful  Description  
test-change-vx7z5  2021-10-06T09:12:36Z  2021-10-06T09:12:36Z  true        update: Op: 1 create, 0 delete, 1 update, 0 noop / Wait to: 2 reconcile, 0 delete, 0 noop  
test-change-hcwrt  2021-10-06T09:11:56Z  2021-10-06T09:11:56Z  true        update: Op: 2 create, 0 delete, 0 update, 0 noop / Wait to: 2 reconcile, 0 delete, 0 noop  

2 app changes

Succeeded

❯ kapp app-change describe -a test --app-change test-change-vx7z5

@@ create configmap/config-1-ver-2 (v1) namespace: default @@
  ...
  1,  1   data:
  2     -   foo1: bar
      2 +   foo: bar
  3,  3   kind: ConfigMap
  4,  4   metadata:
@@ update configmap/config-2 (v1) namespace: default @@
  ...
  8,  8         kind: ConfigMap
  9     -       name: config-1-ver-1
      9 +       name: config-1-ver-2
 10, 10     creationTimestamp: "2021-10-06T09:11:56Z"
 11, 11     labels:

Acceptance Criteria

• [x] Text diffs for app changes should be serialised and stored in app-change config maps in addition to any other information that might add value.
• [x] Expose command app-change describe app_change_name which presents the information for that particular change in an intuitive view.
• [ ] Make a flag --app-change-diff available, which can let the user to specify that they do not want to store the diff for a particular change.

Checks needed

• [x] Text diffs should be truncated if greater than 0.5 MB in size keeping in mind etcd limitations.
• [ ] Consider special cases that might cause noisy diffs. (Like resources with update-strategy: skip)

Anything else you would like to add:

This will provide much needed observability into changes being applied by kapp in scenarios where a kapp deploy is automatically triggered periodically or due to an action.

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[100mik]

We should be allowing users to easily exclude text-diff for certain resources. This could be done by:

• An annotation like `kapp.k14s.io/ignore-app-change-diff.
• Allowing the user to specify conditions in Config.

Does it make sense to have something like this in addition to --app-change-diff flag or should these be replacing it?