feat: add resources handling to kubernetes driver

[MichalAugustyn]

This PR covers #210.

I have tried to add a single resources driver-opt at first which could be used as following:

buildx create --driver kubernetes --driver-opt replicas=1,resources="requests.cpu=100m,requests.memory=100Mi,limits.cpu=100m,limits.memory=100Mi"

But then found out its not possible to nest keyvalues inside driver-opts at the moment (raised an issue for that #617)

The next step could be #377 where it was proposed to add a possibility of extending the YAML manifest.

I might take care of that later, but decided to add resource handling in a simple way for now.

[tonistiigi]

@AkihiroSuda @morlay