Rename DurableOrchestrationClient to DurableClient

[cgillum]

Now that DurableOrchestrationClient supports entity operations in Durable 2.x, we should rename it to DurableClient. This also makes it consistent with the binding type in function.json.

To make this a non-breaking change, we should continue to support DurableOrchestrationClient, but mark it as deprecated. I'm not sure what the official way to do this is in TypeScript, but apparently you do this in JS using the @deprecated JSDoc syntax.

[hossam-nasr]

Discussed this offline. Let's just do this in v3.x (without keeping the old name around), and it's not necessary to do this in v2.x, so closing this issue.