Make it easy to 'inherit' config for filters (listener->vhost->route)

The current model of filter config is we *usually* do a complete override of the parent config when we add a vhost or route config. The reason for this is not an aesthetic or intentional one, but rather that the current implementation makes inheritance-based configuration unwieldy and unperformant. It's also impossible to correctly verify that the merged/inherited config is valid at config-time; we don't have access to the merged config until the filter begins work.

For example, see [FileSystemBufferFilterMergedConfig](https://github.com/envoyproxy/envoy/blob/7974560b04172e21999ecd1103f9960b5d2899d4/source/extensions/filters/http/file_system_buffer/filter_config.h#L60-L102) - a horrifying thing that has to be reconstructed for each filter instance.

The premise of this issue is to make it so that during `createRouteSpecificFilterConfigTyped`, we have access to the parent config protos, and thereby can perform a merge relatively easily, just once, at config-time, including validation.

One possible wrinkle with this idea is in a case where the config looks like
```
listener {
httpfilter{ecds_dynamic_my_filter},
vhost {
route {
httpfilter{my_filter{x:1, y:2}}
}
}
}
```
An update to the dynamic config in this context may not automatically re-call `createRouteSpecificFilterConfigTyped` for the potentially-impacted 'child' filters (e.g. a hypothetical change from z:3 to z:4 should update both versions of the config), so it may be necessary to also explicitly do that as part of this change.