Use cobra cli for docker-generate command

[chloeyin]

Signed-off-by: zaihaoyin zaihaoyin@microsoft.com

What

Fix #183 by using github.com/spf13/cobra to replace github.com/urfave/cli/v2, which is a cli package also used by oras-cli.

Notice

The whole PR will update about 50 files, which is too difficult to review. I will split them into 4 PRs.
I expect these 4 PRs will do different jobs:

1. The first PR will update the docker-generate command to use cobra and add some common flags whichcobra will use in later PRs.
2. The second PR will update the docker-notation command to use cobra.
3. The third PR will update the notation command to use cobra.
4. The forth PR will do some clean up jobs, including remoing unused functions and variables, etc.

This is the first PR of the series.

[priteshbandi]

Are we going to support docker plugin in notation? If not then we can get rid of docker-generate and docker-notation packages.

[SteveLasker]

LGTM for the move to cobra.

[dtzar]

@priteshbandi - I created a new discussion for dropping those commands. I believe it is a good thing to consider. I think we should not hold up this PR though on the outcome of the discussion.

However @chloeyin I do think you should skip the docker-notation PR for now unless you already have it effectively completed (I know the request was in a bigger PR earlier).

[chloeyin]

@priteshbandi - I created a new discussion for dropping those commands. I believe it is a good thing to consider. I think we should not hold up this PR though on the outcome of the discussion.

However @chloeyin I do think you should skip the docker-notation PR for now unless you already have it effectively completed (I know the request was in a bigger PR earlier).

Yes, I can skip the docker-notation PR and keep the docker-notation pkg using github.com/urfave/cli/v2 for now.

[dtzar]

Since the plan from the discussion yesterday is to remove this and we have two code reviews/approvals from our side. I'm going to go ahead and merge it.