[proxyd] support different BackendOptions for different backends within the same config

[hmvien]

Is your feature request related to a problem? Please describe.
We have a proxyd instance with several RPC backends configured with different level of reliability. As a result, we would like to configure different max_latency_threshold and max_error_rate_threshold per backend. The current state is that proxyd accepts a single block for BackendOptions and it will be applied to all backends. We would like more control.

Describe the solution you'd like
A tier-based config parsing logic that will allow the max_latency_threshold and max_error_rate_threshold to take precedence within the backends config block. If this value is not set, then it will fallback to use the backend options

It might be worthwhile to apply this logic to all existing configs in the BackendOptions block.