[FR] Add "metadata=" on `role` helper, add newPart helper.

To allow prompts to annotate themselves with metadata, I propose two additions:

### 1. A metadata hash argument for the role and media helpers

```handlebars
{{role "system" metadata.foo="bar"}}
{{media url="..." metadata.foo="bar"}}
```

**Note:** I believe the `metadata.foo` syntax style is necessary because Handlebars doesn't support object literals. The algorithm should be to iterate through hash arguments for anything beginning with `metadata.*` and add each of those as metadata keys. Metadata should be merged with existing metadata (if any).

### 2. A `newPart` helper.

This helper creates a new text part with optional metadata:

```handlebars
{{newPart metadata.foo="bar"}}
```

This allows the user to create custom "sections" within their prompt for better observability and understanding.