Separates the validator clients more cleanly from the CL clients. This
then allows the use of different combinations of CL/VC clients, e.g.
Teku VC with Lodestar CL. The only VC that doesn't work with different
beacon nodes is Prysm at the moment.

The `use_separate_validator_client` flag defaults to false for CL
clients that can run validators in the same process as the CL
(preserving the way the `cl_split_mode_enabled` worked before this PR).

I believe this can be quite useful to test different VC<->CL
combinations for compatibility.

---------
Co-authored-by: Barnabas Busa <busa.barnabas@gmail.com>