feat(cli): enable extended client access

[anpep]

This patch gets rid of the clientMixin in command structs in favor of passing a CmdOptions struct to the New (formerly Builder) function in CmdInfo.

Example usage:

Command implementation

type cmdFoo struct {
        /* ... */
	client *MyClient
}

func init() {
	AddCommand(&CmdInfo{
		/* ... */
		New: func(opts *CmdOptions) flags.Commander {
			return &cmdFoo{client: &MyClient{opts.Client}}
		},
	})
}

func (cmd *cmdFoo) Execute(args []string) error {
        return cmd.client.Foo()
}

Client implementation

import "github.com/canonical/pebble/client"

type MyClient struct {
        *client.Client
}

func (c *MyClient) Foo() error {
        /* ... */
}

[benhoyt]

Just a comment about the name.

I'd be fine with this. However, are you able to link to the previous discussion where I pushed back on the whole concept of the clientMixin? I can't find that right now, and I'm interesting in going back over my thoughts.

[paul-rodriguez]

I prefer the new naming.

[flotter]

Looks great. Few super nitty questions / comments.

[paul-rodriguez]

Great job !

[benhoyt]

Yeah, I like this -- it's a tiny bit more verbose, but it seems much more direct and Go-like than the "mixin" concept.

[niemeyer]

This turned out very well, thank you. Only minor comments below and LGTM with these considered.

[flotter]

Discussed final state with Gustavo and got go-ahead to merge.