Add a Functions-native API for invoking Durable Workflows

[greenie-msft]

Description

Invoking a Durable Workflow from an Azure Function currently requires manually creating an HttpClient, constructing the workflow endpoint URI, and making an HTTP request.

This introduces unnecessary boilerplate and couples in-process function code to the workflow's HTTP surface.

Proposed experience

Provide a first-class Functions-native client or binding that can invoke a registered Durable Workflow by name, with typed input, without requiring an endpoint URI or HttpClient.

Acceptance criteria

• A function can start a registered Durable Workflow through an injected client or binding.
• Callers do not need to construct URLs or HTTP requests.
• The API returns a workflow instance handle or identifier.
• Invocation works independently of whether an HTTP trigger is exposed.

[kshyju]

We already have a IWorkflowClient which we should make available to the customer code via DI container. (Currently it is exposed in the console app model only). We may need to expose another overload of RunAsync which takes the workflow name.

[greenie-msft]

Closing. Completed in: [.NET] Add a Functions-native API for invoking Durable Workflows #48