Add support for user defined port forwarding

[priyawadhwa]

This PR adds support for user defined port forwarding and updates docs around it.

Users can now add something like the following to their skaffold config:

portForward:
- resourceType: deployment
  resourceName: myDep
  namespace: mynamespace
  port: 8080
  localPort: 9000

This PR adds validation for resourceType and sets defaults on namespace and localPort if not specified in the config. Also adds unit tests to test for each of these cases.

[container-tools-bot]

Please visit http://35.236.53.50:1313 to view changes to the docs.

[codecov]

Codecov Report

Merging #2336 into master will increase coverage by 0.12%.
The diff coverage is 86.88%.

Impacted Files	Coverage Δ
pkg/skaffold/schema/latest/config.go	100% <ø> (ø)	⬆️
pkg/skaffold/schema/validation/validation.go	97.56% <100%> (+0.68%)	⬆️
pkg/skaffold/runner/dev.go	63.38% <100%> (ø)	⬆️
pkg/skaffold/schema/defaults/defaults.go	90.41% <100%> (+0.55%)	⬆️
pkg/skaffold/runner/runner.go	68.46% <100%> (+0.24%)	⬆️
...g/skaffold/kubernetes/portforward/pod_forwarder.go	87.32% <100%> (+0.18%)	⬆️
...affold/kubernetes/portforward/forwarder_manager.go	31.57% <50%> (ø)	⬆️
...ffold/kubernetes/portforward/resource_forwarder.go	52% <58.33%> (-0.09%)	⬇️
...g/skaffold/kubernetes/portforward/entry_manager.go	93.1% <66.66%> (-6.9%)	⬇️

[balopat]

LGTM

[container-tools-bot]

Please visit http://35.236.70.48:1313 to view changes to the docs.