Unable to use kustomize remote resources

[ron1]

As described here, kustomize supports the use of remote/external resources for builds. Fleet should also support using remote/external resources for kustomize workloads much like it does for helm workloads as demonstrated in example helm-external.

Reproduce the bug using the following steps:

1. Download fleet v0.3.8 binary and add it to your path
2. Git clone git repo https://github.com/ron1/fleet-examples branch issue-697-kustomize-unable-to-use-remote-resources
3. Change directory into fleet-examples/multi-cluster/kustomize-external
4. Invoke fleet test -t dev to build the dev overlay

Expected output: Generate the expected kustomize build output using the following steps:

1. Download kustomize v4.2.0 binary and add it to your path
2. Follow steps 2 and 3 above
3. Invoke kustomize build overlays/dev

Actual output: The following error is displayed:

# Matched: dev
FATA[0001] error while running post render on files: accumulating resources: accumulation err='accumulating resources from '../../base': 'base' must resolve to a file': recursed accumulation of path 'base': accumulating resources: accumulation err='accumulating resources from 'https://github.com/rancher/fleet-examples/multi-cluster/kustomize/base': yaml: line 216: mapping values are not allowed in this context': '/tmp/kustomize-001523165/multi-cluster/kustomize/base' doesn't exist