Implement inlining content of external file to policy

### Proposal

It would be beneficial to be able to inline external files in policy document. It would allow maintaining those files separately from policy.

Example of C# code:
```csharp
context.SetBody(File.ReadAllText("body.liquid"), new SetBodyConfig() { Template = "liquid" });
```

Compiled to
```xml

{% if user %}
Hello {{ user.name }}!
{% endif %}

```

### Component

Authoring library, Compiler