Create a bundle or image directly as a tar file

**Describe the problem/challenge you have**
In a scenario where the artifact that will be shared is a tar, there might not be a need to use the registry as an intermediary step. This would allow the users to combine the commands `imgpkg push` + `imgpkg copy --to-tar` in a single command and without storing extra images in a registry

**Describe the solution you'd like**

Scenario for images
**Given** a set of files in examples/basic-step-1
**When** execute `imgpkg build --tag v0.5.0 -f examples/basic-step-1 --to-tar some.tar`
**Then** copy the image to a registry`imgpkg copy --tar some.tar --to-repo localhost:5000/some-image`
**And** retrieve it using `imgpkg pull -i localhost:5000/some-image:v0.5.0`

Scenario for bundles
**Given** a bundle in examples/basic-step-2
**When** execute `imgpkg build --tag v0.5.0 -f examples/basic-step-1 --to-tar some-bundle.tar`
**Then** copy the bundle to a registry`imgpkg copy --tar some-bundle.tar --to-repo localhost:5000/some-bundle`
**And** retrieve it using `imgpkg pull -b localhost:5000/some-image:v0.5.0`

Notes:
- Tags used during the build step should be kept when copying

Based on a request from Zihan Jiang: [Slack conversations](https://kubernetes.slack.com/archives/CH8KCCKA5/p1610484485443500)