Checkpoint sync default behaviour for testnets

### Problem description

This issue is being created to follow up on the [comment](https://github.com/ChainSafe/lodestar/pull/6054#discussion_r1370119474) in #6054 (ephemery testnet config) relating to checkpoint sync behaviour. [Ephemery testnet spec](https://github.com/taxmeifyoucan/EIPs/blob/d298cdd8eaf47a21e7770e5c6efef870587c924d/EIPS/eip-6916.md) requires _automatic_ use of checkpoint sync URL, unless overridden by the user with an alternative endpoint.

In Lodestar, however, the checkpoint sync URL has to be provided together with the `--checkpointSyncUrl` flag. Using the checkpoint sync URL is therefore not current "default" behaviour for testnets.

The purpose of this issue is to open discussion on the preferred default behaviour for ephemery, and other testnets, to obtain the genesis state.

### Solution description

Options are discussed below.

If changing default behaviour for testnets:
- add new property for each network `checkpointSyncUrl` (string), but keep it undefined for mainnet
- add flag `--disableCheckpointSync` (bool)
Note: For mainnet, a user will still have to choose the checkpoint sync provider they want to use (or none at all).

Otherwise:
- retain the `--checkpointSyncUrl` flag as it is currently implemented, and ensure this is properly documented as ephemery recommended usage.

### Additional context

_No response_