chore: adding more required functionality

[priteshbandi]

• Adding more functionality required for notation-go to depend on this pkg.
• Also some renaming and update to Error struct to matching it with notation-go.

[Two-Hearts]

1. Please add doc to struct CLI defined at line 23 of cli.go.
2. When I new a CLI struct with pl == nil: cli, _ := New("notation-my-plugin", nil), this will panic when I call cli.Execute(...). We should always check nil when new a CLI.
3. In NewWithLogger function, we are only checking prefix of the plugin executable name. We should also check the suffix matches with the plugin's metadata.Name, i.e. {plugin-name} of notation-{plugin-name} should equal to pl.GetMetadata resp.Name. In fact, do we really need the executableName parameter here?
4. deferStdout() and type discardLogger struct{} should be moved to utils.go
5. In utils.go getValidArgs(), the result strings should not be hardcoded, because they are defined in the proto.go:

   notation-plugin-framework-go/plugin/proto.go

   Line 27 in c8c68d2

   type Command string

[priteshbandi]

1. Please add doc to struct CLI defined at line 23 of cli.go.

   2. When I new a CLI struct with pl == nil: cli, _ := New("notation-my-plugin", nil), this will panic when I call cli.Execute(...). We should always check nil when new a CLI.

   3. In NewWithLogger function, we are only checking prefix of the plugin executable name. We should also check the suffix matches with the plugin's metadata.Name, i.e. {plugin-name} of notation-{plugin-name} should equal to pl.GetMetadata resp.Name. In fact, do we really need the executableName parameter here?

   4. deferStdout() and type discardLogger struct{} should be moved to utils.go

   5. In utils.go getValidArgs(), the result strings should not be hardcoded, because they are defined in the proto.go:

      notation-plugin-framework-go/plugin/proto.go

      Line 27 in c8c68d2

      type Command string

Done. Addressed all the feedbacks

[Two-Hearts]

LGTM with a single nit comment.

[Two-Hearts]

LGTM

[rgnote]

LGTM

[JeyJeyGao]

LGTM