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

[mbleigh]

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

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

{{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:

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

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