adds `env` in shellpod

[wonko]

Allows to define env vars for the shellpod container.

Closes https://github.com/derailed/k9s/issues/2244

  shellPod:
    image: busybox:1.35.0
    env:
    - name: HELLO
      value: world
    namespace: default

results in

<<K9s-Shell>> Pod: default/k9s-shell-3745 | Container: k9s-shell 
/ # set | grep HELLO
HELLO='world'
/ # 

[wonko]

@wonko Thank you for this update Bernard! We should update the docs as well to make sure users are aware.

sure, if you can point me to where the docs are, i can try to update the content to include the env section on the shellpod

[derailed]

@wonko Thank you! Sure you can update the README.md for the time being. Also there are some conflicts ;(

[wonko]

@derailed updated the readme (and merged master).