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.