Allow forward of undiscovered ports

[bonelli]

Is your feature request related to a problem? Please describe.
When I try to configure a TCP port forward for a pod, if the Target port is not listed it is impossible to force it to a value.

Describe the solution you'd like
I don't know how the autodiscovery of open port is done, but in this particular case I have a dynamic port opened occasionally on a development pod, I want to forward it and I can't.
I simply wish it was possible to force a Target port even if it is unlisted.

Here an image of my case

[hcavarsan]

@bonelli added this in v0.15.7

you can now select "custom port..." in the dropdown and enter any port number manually.

let me know if that works for you