Show what's updated in "update" command (or its invocations indirectly)

[ahmetb]

This is something Homebrew has, and I've been looking to have it in Krew, too.

When you run krew update (or indirectly, through krew install or krew upgrade), you'll see a message: Updated the local copy of plugin index. This message is nice, but it doesn't provide the big picture.

I think for update command specifically, it would be good if we can show something like:

$ kubectl krew install foo
\
 │ Updating the local copy of plugin index.
 │   New plugins available:
 │     * multiwatch v0.1.0
 │     * view-secret v0.1.9
 │   The following plugins have new version:
 │     * whoami v0.3.2 -> v0.4.0
 │     * rbac-lookup v1.0.2 -> v1.0.3
 │     * get-all v0.8.0 -> v0.9.1  🚨 (a sign to indicate it's installed)
/
Installing plugin "foo".
[...]

Some things to consider:

• requires significant change to underlying update mechanism, as we need to parse all index, store in-memory, then compare (worth it?)

• the above example could be too much output for the install command, maybe we figure out a one-liner version like:

   New plugins available: multiwatch v0.1.0, multiwatch v0.1.0
   Upgrades available: get-all v0.8.0 -> v0.9.1 <--- 🚨only installed ones
  

• this should probably be tested only as unit tests as it would be unnecessarily complex to make integration tests for the update scenarios

I'm supportive of this change only if it can be done with small effort.

/priority P3
/priority backlog
/good-first-issue

[artmello]

Linked PR implements a prototype for this feature. Above some example outputs of it so suggestions can be made about how to improve it.

When nothing was changed on index

./out/bin/krew-darwin_amd64 update
Updated the local copy of plugin index.

When system has following Name/Version information on index:

"foo":  "v0.1.0",
"bar":  "v0.1.0",
"fizz": "v0.1.0",
"buzz": "v0.1.0",

and receive following information after update:

"new-foo": "v0.1.0",
"new-bar": "v0.1.0",
"foo":     "v0.1.0",
"bar":     "v0.1.0",
"fizz":    "v1.1.0",
"buzz":    "v1.1.0",

With fizz plugin being already installed on the system

./out/bin/krew-darwin_amd64 update
Updated the local copy of plugin index.
  New plugins available:
    * new-bar v0.1.0
    * new-foo v0.1.0
  The following plugins have new version:
    * fizz v0.1.0 -> v1.1.0 (!)
    * buzz v0.1.0 -> v1.1.0

Both lists are sorted by Name (with installed plugins showing up first for the new versions list. By now this is the same output for all commands that call update. If we agree this is the desired approach, I plan to work on a simplified version of such output for command that call update as a pre requisite (i.e install).

PR is missing unit tests and failing on integration tests right now