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

**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:
```zsh
❯ 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](https://github.com/kubernetes/kubernetes/issues/19781).
- [ ] 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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