Add DurableClient class declaration

[hossam-nasr]

Resolves #154, part of #416. Adds a DurableClient class declaration that the DurableOrchestrationClient class implements, and exports it from the root of the package.

// before
const client = df.getClient(context)
// client. -> no intellisense

// after
const client: DurableClient = df.getClient(context);
// client. -> all the intellisense ✨

[davidmrdavid]

LGTM. From the discussion, it would appear we are ok with this change.