Allow specifying context for kubernetes driver

Description

Currently the kubernetes driver searches for the builder deployments in the namespace in the current context.

Instead it should support looking for the deployments in a named context.

This would allow a use case where the current context is the application context, and a different context can be used strictly for builders.

For example we can do it by specifying the context name in the kuberenetes endpoint like so

kubernetes://context-name/kube?deployment=builder-amd64\u0026kubeconfig=

Or in the driver options

"DriverOpts":{"context":"context-name","namespace":"buildkit","nodeselector":"kubernetes.io/arch=arm64"}

EDIT: I know that I can specify a separate config file which will only contain the builder context, but I rather use the default config with all my contexts, currently that is the workaround that I use.

Locate the Kubernetes driver handling for the kubernetes:// endpoint and DriverOpts, then trace how the current context is selected for builder deployments. Compare both proposed context forms and verify that a named builder context can be used while the application context remains current. Done means the named context works with the existing namespace, node selector, and kubeconfig options.