Export package from SAML-authed kibana

@ycombinator mentioned to me that you could use the `export` subcommand against a non-local kibana via environment variables:

```
export ELASTIC_PACKAGE_ELASTICSEARCH_HOST=http://127.0.0.1:9200
export ELASTIC_PACKAGE_KIBANA_HOST=http://127.0.0.1:5601
export ELASTIC_PACKAGE_ELASTICSEARCH_USERNAME=...
export ELASTIC_PACKAGE_ELASTICSEARCH_PASSWORD=...
```

But only if basic auth is supported.

It'd be good to have a case to cover kibanas that are authed via SAML (or possibly other mechanisms).

Some thoughts I had that might help here:

- have something in the kibana UI handle the via the browser
- guide the user to provision/use API keys