Add --additional-tag option to imgpkg push.

**Describe the problem/challenge you have**

I need to be able to add multiple tags to an image when using `imgpkg push -i`. For example, I would use an explicit tag based on version or a sha, but at the same time still need the image tagged with `latest`. At present need to do two runs of `imgpkg push` with the same input.

```
imgpkg push --file . \
--image ghcr.io/${REPOSITORY}:sha-${REPOSITORY_SHA7} \
--registry-username=${{github.actor}} \
--registry-password=${{secrets.GITHUB_TOKEN}}
imgpkg push --file . \
--image ghcr.io/${REPOSITORY}:latest \
--registry-username=${{github.actor}} \
--registry-password=${{secrets.GITHUB_TOKEN}}
```

Running `imgpkg push` twice is inconvenient.

**Describe the solution you'd like**

Add an `--additional-tag` option for allowing one or more (list option more than once) tags to the image in addition to the primary tag in the image reference given to `--image` option.

**Anything else you would like to add:**

Since `imgpkg push` already adds a `sha256-???.imgpkg` tag in addition to the user tag, the mechanics should exist already to do this, so shouldn't be a huge problem.

The name for the option of `--additional-tag` is inspired by option of same name in `skopeo` used to add extra tags when copying images.

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