Allow historically "core" clusters to use custom cluster configuration

Title: Allow historically "core" clusters to use custom cluster configuration

Description: Custom clusters as an extension point were added in https://github.com/envoyproxy/envoy/issues/5780. As part of slimming down the binary for EM, we moved "traditional" clusters to extensions e.g. in https://github.com/envoyproxy/envoy/pull/23920, https://github.com/envoyproxy/envoy/pull/23890, etc.

While some of the historically core clusters can be configured via custom cluster type in

https://github.com/envoyproxy/envoy/pull/6118/files#diff-cc3502882065c5e035482a2bccd3a7e43ba62017ad01e4ebb7acb2e15fd31075R113-R114

It works for static clusters, but it doesn't work for eds clusters -- instead EDS clusters rely on using the traditional configuration when it was part of "core". We'd expect CustomClusterType configuring an EDS cluster to "just work" with EDS clusters by having the config packed into the CustomClusterType typed_config but it gets rejected during validation.

This may apply to other "historical" core cluster types as well.