Support starting a new orchestration from within orchestrators

Usually, orchestrations are only to be invoked by a durable client, but in dotnet users have the ability to start orchestrations within another orchestrations.

The dotnet API for this is here: https://docs.microsoft.com/en-us/dotnet/api/microsoft.azure.webjobs.extensions.durabletask.idurableorchestrationcontext.startneworchestration?view=azure-dotnet#microsoft-azure-webjobs-extensions-durabletask-idurableorchestrationcontext-startneworchestration(system-string-system-object-system-string)

We should implement similar functionality

Review the TypeScript orchestration context entry points and compare them with the linked .NET startNewOrchestration API. Define the JavaScript-facing behavior and acceptance checks for starting a child orchestration from an orchestration; the issue names no files or tests, so repository exploration is required.

Written by the indexing model from the issue text.