Support set-json for helm

Skaffold uses a very magicy yaml library.
This makes it often very hard/impossible to pass boolean/numeric values to helm as a strings.

Supporting passing raw (uninterpreted) json values to `helm` via `--set-json` should fix that.

And example could be:

```yaml
manifests:
helm:
releases:
- name: foo
setJsonValues: {port: "345"}
```

This of course only works if one can force the yaml library to handle values of `setJsonValues` always as strings.
Alternatively one could add an escape hatch via a release-specific `flags`:

```yaml
manifests:
helm:
releases:
- name: foo
flags:
- --set-json={"port": "345"}

```