Mark docker app plugin as experimental

[silvin-lubecki]

- What I did

• Mark the app plugin as an experimental plugin.

- How to verify it

1. Use this docker/cli
2. Build the docker app plugin
3. Move it to your ~/.docker/cli-plugins directory
4. $ docker --help -> app does not appear in the manage commands
5. $ DOCKER_CLI_EXPERIMENTAL=enabled docker --help -> app appears in the manage commands

- Description for the changelog

• app plugin is now experimental and only shown if the docker/cli has its experimental flag enabled.

- A picture of a cute animal (not mandatory but encouraged)

[silvin-lubecki]

@simonferquel can you review the docker desktop context store part, not really sure that what I did is right 🤔

[codecov[bot]]

Codecov Report

Merging #537 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #537   +/-   ##
=======================================
  Coverage   69.94%   69.94%           
=======================================
  Files          55       55           
  Lines        3101     3101           
=======================================
  Hits         2169     2169           
  Misses        639      639           
  Partials      293      293

Impacted Files	Coverage Δ
cmd/docker-app/main.go	63.63% <ø> (ø)	:arrow_up:

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