ci/cd: consider exporting some go routines so other projects can ci/cd against notation

[rchincha]

For example,

https://github.com/notaryproject/notation/blob/main/cmd/notation/cert_gen.go

[sajayantony]

Should this issue be in https://github.com/notaryproject/notation-go-lib

[rchincha]

notaryproject/notation#141
^ here is a draft PR that could help.
Would be even better if some routines can be pulled out without tying to cli context.

[rchincha]

Should this issue be in https://github.com/notaryproject/notation-go-lib

^ looks a little low-level for the use case in mind. I would have to re-implement signer and verifier perhaps.

[SteveLasker]

Hi @rchincha,
Thanks for the PR and Issue. We're hoping to keep the CLI focused on core CLI scenarios, with minimal surface area. The functionality you need, should be enabled through the go-libraries. But, this does bring up an interesting point for how the notation plug-ins will work.
I'd like to transfer this to the notation-go-lib for tracking. I'll open an issue related to using plug-ins, when using the notation-go libraries.