[Key Vault] Expose a send_request method on KV clients

[mccoyp]

Using DPG, we should add a send_request method to KV clients so that clients can send requests that there's no specific API surface for, using the same auth and pipeline that other client requests do. This would be particularly helpful for cases when the Azure CLI needs to send requests for new service features via the Python SDK.

[mccoyp]

Update to this: there should actually already be a send_request method on generated clients. (DPG isn't currently applicable to KV because we use the multi-API generation script with autorest). We just need to expose this method on top-level clients.

[mccoyp]

Update (again): there are _send_request methods on generated clients, but only on API version-specific clients (example). However, since KV uses multi-API generation, our top-level clients don't actually use these clients and instead are built on the multi-API client -- which doesn't have this method.

The implementation is very straightforward, so we can just implement send_request ourselves for now. But once the generated method is available on a client that we actually use, we should probably switch to using its implementation.

[mccoyp]

Resolved by #29124