Support managing plugin keys

[qmuntal]

This PR adds support for:

• Listing available plugins using notation plugin list:

> notation plugin list  
NAME   DESCRIPTION   VERSION   URL           SUPPORTED CONTRACTS   CAPABILITIES   ERROR                                                                      PATH
foo    friendly      1         example.com   [1]                   [cap]          executable name must be "notation-foo.exe" instead of "notation-baz.exe"   C:\Users\qmuntaldiaz\.notation\plugins\baz\notation-baz.exe
foo    friendly      1         example.com   [1]                   [cap]          <nil>                                                                      C:\Users\qmuntaldiaz\.notation\plugins\foo\notation-foo.exe

• Adding plugin keys using notation key add --plugin=foo --name=foo-key --id=foo-id.
• Listing plugin keys using notation key list:

> notation key list
NAME      KEY PATH   CERTIFICATE PATH   ID       PLUGIN NAME   
foo-key                                 foo-id   foo

• Removing plugin keys using notation key remove.

Other changes:

• Simplified the types in pkg/config and reduced the API surface.
• Using go 1.18 Generics to operate on slices.
• Improved how we are formatting lists to the standard output.

@gokarnm @SteveLasker

Signed-off-by: qmuntal qmuntaldiaz@microsoft.com

[SteveLasker]

👍 on the new notation plugin list output

[gokarnm]

LGTM. Have a couple of minor comments. I'd ask a review from @shizhMSFT too as he wrote most of this code.

[shizhMSFT]

I've also reopened some comments in the previous review as they are marked resolved without changes.

[qmuntal]

I've also reopened some comments in the previous review as they are marked resolved without changes.

@shizhMSFT I think I got them all now.

[shizhMSFT]

LGTM with one unresolved comment.

[qmuntal]

LGTM with one unresolved comment.

Resolved