Backup feature of the keystore

[tropicar]

Is your feature request related to a problem? Please describe.

Now we have this structure of beacon chain + validator + web3signer , the web3signer stores all the keystores so the old way we did the backups by downloading the backup from the validator container that contained the keystone now is not possible because the keystone is not there, they keystore are in the web3signers.

Describe the solution you'd like

I am not an expert on how the web3signer works, but if the keystore is allocated on web3signer, we should be able to do a backup of this keystores and restore it in the same way we did in the past with the validator service.

Additional context

We should have some easy method of doing backup and restore.