support more dynamic cluster header

*Title*: *support formatted cluster header*

*Description*:

[cluster_header](https://www.envoyproxy.io/docs/envoy/latest/api-v3/config/route/v3/route_components.proto#envoy-v3-api-msg-config-route-v3-routeaction) is used to select dynamic upstream cluster based on the request headers.

This function is very useful. But I would like to make this feature more flexible with a `core.v3.TypedExtensionConfig` API. Like being able to select an upstream cluster based on a number of different headers, or formatting the header values and selecting the corresponding upstream cluster, etc.