ux: Support command docker extension update --install

[sandipchitale]

Is your feature request related to a problem? Please describe.
Many times during iterative development of an extension one may manually uninstall the extension. After that the next command has to use

docker extension install ...

and the repeated:

docker extension update ...

I keep forgetting to up arrow in the shell and change update to install from the previous repeat cycle and then next time switch back 'install' to 'update' again.

Describe the solution you'd like

Support option like

docker extension update --install ...

Especially because the syntax is pretty much the same for install and update.

This request is somewhat analogous to the helm commands option:

`

Upgrade helm release but if absent install

helm upgrade --install ...
`

Hope this makes sense.

Describe alternatives you've considered

Well I can always have two different aliases and remember to use the correct one.

Additional context

[sandipchitale]

I am using something like this for now:

docker extension $(docker extension ls | grep --silent kubernetes-dashboard ; if [  $? -eq 0 ] ; then echo 'update' ; else echo 'install' ; fi)  sandipchitale/kubernetes-dashboard:1.0.0 -f