Add jsonnet native function (manifestYamlFromJson)

Describe the bug/feature When I use Grafana's ksonnet-util 1 with Kapitan, I can't do kapitan compile because that Jsonnet native function(manifestYamlFromJson) is null.

vendor/ksonnet-util/util.libsonnet:206:5-27       function <anonymous>
...
RUNTIME ERROR: only functions can be called, got null

ksonnet-util/util.libsonnet:

204   manifestYaml(value):: (
205     local f = std.native('manifestYamlFromJson');
206     f(std.toString(value))
207   ),

To Reproduce

jb init
jb install github.com/grafana/loki/production/ksonnet/promtail
jb install github.com/jsonnet-libs/k8s-alpha/1.16

$ cat components/promtail/main.jsonnet
local promtail = import 'promtail/promtail.libsonnet';

promtail {
  _config+:: {
    namespace: 'addon-logging',
  },
}

$ kapitan compile

Jsonnet error: failed to compile /Users/anarch/git/anarcher/k8s-kapitan-addons/components/promtail/main.jsonnet:
 RUNTIME ERROR: only functions can be called, got null
        /Users/anarch/git/anarcher/k8s-kapitan-addons/vendor/ksonnet-util/util.libsonnet:206:5-27       function <anonymous>
        /Users/anarch/git/anarcher/k8s-kapitan-addons/vendor/promtail/promtail.libsonnet:41:23-61       object <data>
        /Users/anarch/git/anarcher/k8s-kapitan-addons/vendor/github.com/jsonnet-libs/k8s-alpha/1.16/_gen/core/v1/configMap.libsonnet:59:27-31  object <anonymous>
        /Users/anarch/git/anarcher/k8s-kapitan-addons/vendor/promtail/promtail.libsonnet:(39:5)-(42:7)  object <anonymous>
        During manifestation

Compile error: failed to compile target: dev

Expected behavior A clear and concise description of what you expected to happen.

Screenshots If applicable, add screenshots to help explain your problem/idea.

If it's a bug (please complete the following information):

python --version:
pip3 --version:
Are you using pyenv or virtualenv?
```
$ kapitan --version
0.29.5
```

Additional context

This manifestYamlFromJson func is defined in Tanka's native funcs: https://github.com/grafana/tanka/blob/17da685bbff3baee75e529b13c19d8f047015395/pkg/jsonnet/native/funcs.go#L109

kapicorp / kapitan

Add jsonnet native function (manifestYamlFromJson) #729