[Feature] notation login

[SteveLasker]

The current notation cli supports -u / -p for each command. However, having to provide the username/password for each command is tedious at best.
Alternatively, cli commands provide a notation login -u -p type of experience where the subsequent commands can forgo having to provide the commands each time.

Ideally, this would be provided by the oras-go library, and notation just benefits from that common experience.

• auth should support encrypted storage or usernames and passwords, possibly following the docker encryption story
• not be dependent on the docker client or libraries being installed
• support basic auth flows that use cloud provider tokens for native authentication
• support 2fa cloud provider capabilities

[shizhMSFT]

Current notation CLI has only a very basic auth / credential module. This will eventually be resolved by moving to the oras-go library.

[SteveLasker]

@shizhMSFT will you be able to integrate this into RC1?
Instead of having to set NOTATION_USERNAME and NOTATION_PASSWORD, we can pickup the docker cred store.

[shizhMSFT]

Login functionalities are currently not present in oras-go. Let me estimate if it fits the RC1 plan.

[gokarnm]

@shizhMSFT were you able to look into #170, basic auth with notation sign $IMAGE -u <USERNAME> -p <PASSWORD> seems to be broken, will doing a new release with ORAS integration changes fix this issue?

[shizhMSFT]

Yes, a new release of notation should fix it.

[gokarnm]

Just clarifying, a new release of notation alpha, without any code changes should fix this issue? Some ORAS integration work was done in Jan (#150).