Handlebar helpers should have access to gRPC metadata

[ledouxpl]

Is your feature request related to a problem? Please describe.
I am unable to perform request matching on gRPC metadata as I am able to do for a HTTP request.

Concretely, I am unable to provide a localized response by honouring the accept-language header which we pass through gRPC metadata.

Describe the solution you'd like
Handlebar helpers should have access to the gRPC request metadata.

Describe alternatives you've considered
Considered modyfing GrpcParser.ts to forward to Handlebar's context the grpc metadata as such:

const fileContent = await template({ request: call.request, metadata: call.metadata });

Then modyfing RequestHelper.ts to provide support for them in the capture helper as such:

case "metadata":
    return context.data.root.metadata.get(context.hash.key);

Enabling me to create a gRPC mock as:

{{#is 'en-US' 'in' (capture from='metadata' key='accept-language') }}
{
    "id": {{num_between lower=500 upper=600}},
    "title": "from matching header!"
}
{{else}}
{
    "id": {{num_between lower=500 upper=600}},
    "title": "no matching header :(",
}
{{/is}}

[github-actions]

🎉 This issue has been resolved in version 0.10.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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