Enabled shared data directory with eth2
When passing `--data-dir`, we want to ensure that eth1 and eth2 can share the same data directory - this requires reviewing where the databases and various configuration files are located and how they are named.

In particular, options like era files, node files, JWT secrets etc have defaults that are relative to `data-dir` and these should be set in such a way that when the option is used by both eth1 and eth2, they should have compatible defaults

* JWT should use the same name for the file so that on startup, it doesn't have to be specified
* the database [should not have unnecessary nesting](https://github.com/status-im/nimbus-eth1/issues/1348)
* the database folder name should be based on the network name (`mainnet`, `sepolia` etc)
