Allow configuring PowerShell edition (powershell vs pwsh) when shelling into pods

[Taha-cmd]

Is your feature request related to a problem? Please describe.
Currently, k9s uses powershell by default as an entrypoint for all Windows containers. (see https://github.com/derailed/k9s/blob/master/internal/view/pod.go#L34). However, this doesn't work for PowerShell Core, which has a different executable name (pwsh).

Describe the solution you'd like
Allow overriding the entrypoint command in the config file. We can keep powershell as the default to remain backwards compatible.