Conditional "per_filter_config"

In a multi-tenant scenario, for the same route, the same filter, different tenants may need to use different route-level filter configurations. Although #4704 can well classify route configurations based on HTTP attributes, it is too complex and heavy for scenarios where only filter configuration is different.

In addition, consider another scenario. Assume we have ten filters and two filters (filters A and B) of them that need to use different route-level config depending on the HTTP attributes. If we don't want to modify the code of filters A and B, then we have to create a lot of match&route to cover all cases. Although the other eight filters except A and B don't have any special configuration, their configuration must be copied.

If the filter can be configured with different route-level configurations depending on the HTTP attributes, then the above two problems can be solved better.