Add support to handle additional OCI tags from user input

[phenixblue]

These changes add the ability to specify additional OCI tags to be pushed when using imgpkg.

It adds the --additional-tags flag to imgpkg push command and allows the user to specify a comma separated list of additional tags, or the flag can be reused multiple times. This flag functions for both images and bundles.

Comma Separated List

$ ./imgpkg push -i jmsearcy/test-imgpkg-image -f ./pkg/imgpkg/cmd/test_assets/image_with_config --additional-tags 1.0.1,1.0.2

TEST - REGISTRY: index.docker.io
TEST - REPOSITORY: jmsearcy/test-imgpkg-image
TEST - NAME: index.docker.io/jmsearcy/test-imgpkg-image:latest
TEST - TAG: latest
dir: .
file: config.yml
dir: .
file: config.yml
dir: .
file: config.yml
Tags: 1.0.1, 1.0.2, latest
Pushed:
index.docker.io/jmsearcy/test-imgpkg-image@sha256:7b77aebdd49c9d3babf251703ce8cc660cf0fd040aa5d29f262d2084e2d59e8a

Succeeded

Multiple flags

$ ./imgpkg push -i jmsearcy/test-imgpkg-image -f ./pkg/imgpkg/cmd/test_assets/image_with_config --additional-tags 1.0.1 --additional-tags 1.0.2

TEST - REGISTRY: index.docker.io
TEST - REPOSITORY: jmsearcy/test-imgpkg-image
TEST - NAME: index.docker.io/jmsearcy/test-imgpkg-image:latest
TEST - TAG: latest
dir: .
file: config.yml
dir: .
file: config.yml
dir: .
file: config.yml
Tags: 1.0.1, 1.0.2, latest
Pushed:
index.docker.io/jmsearcy/test-imgpkg-image@sha256:7b77aebdd49c9d3babf251703ce8cc660cf0fd040aa5d29f262d2084e2d59e8a

Succeeded

Fixes #355

[joaopapereira]

Some nitpicks and a question.
Do you think it would also make sense to add 1 end 2 end test to ensure all is good?

[phenixblue]

I'm not familiar with the end-to-end tests. Can you point me at an example or describe what that might look like?

[joaopapereira]

I'm not familiar with the end-to-end tests. Can you point me at an example or describe what that might look like?

Maybe we can use https://github.com/carvel-dev/imgpkg/blob/develop/test/e2e/push_test.go as a base. Just making sure that the correct tags are added to the file and also in the registry (https://github.com/carvel-dev/imgpkg/blob/develop/test/e2e/copy_from_lockfiles_test.go#L226), this last example is on how can we check that multiple tags are present in the registry but the rest of the test is a copy test which we do not want to do.

[phenixblue]

Ok, I'll take a look and give it a shot

[phenixblue]

I've not been able to look at this much in the past few months due to a job change, but will be getting back to it soon