Improve usability of the gRPC rate limiter

[rleungx]

Enhancement Task

Previously, we introduced #6834 to offer a self-protection mechanism for PD itself. But it only allows using the HTTP API to change the config which is inconvenient. We need to support PD control to manage the configuration.
Besides, after we support the gRPC rate limiter, the error returned will be regarded as the same as the other errors and the client will back off. And pd client doesn't support custom backoff.
We can support configuring custom backoff internally on pd client side to handle different errors so that the caller doesn't need to care about it.