Improve support for external files in the bundle

[Heitor]

It would be very handy to have charms configuration options as a separate file from the bundle file.

Juju supports a `include-file` directive, but it is limited to reading the contents as a string with a `\n` at the end, with errors when a charm expect an int for example:

```
$ cat bundle.yaml
applications:
  grafana:
    charm: grafana
    options:
      port: include-file://config

$ cat config
1234

$ juju deploy --verbose ./bundle.yaml
Executing changes:
- upload charm grafana from charm-hub with architecture=amd64
- set application options for grafana
  setting options:
    port: "1234\n"
Deploy of bundle completed.
```

Grafana errors out.

Another limitation is that this is not possible:
```
$ cat bundle.yaml
applications:
  grafana:
    charm: grafana
    options: include-file://config

$ cat config
port: 1234
anonymous: true

$ juju deploy ./bundle.yaml
ERROR cannot deploy bundle: cannot unmarshal bundle contents: unmarshal document 0: yaml: unmarshal errors:
  line 3: cannot unmarshal !!str `include...` into map[string]interface {}
```

Initial discussion on:
https://discourse.charmhub.io/t/how-to-manage-a-charm-bundle/1058/12

[Ian Booth (wallyworld)]

We don't plan on working on this in the next milestone so will remove from the milestone for now.