Support connecting multiple external signers to validator client

[nflaig]

Problem description

Depending on demand, we could consider support for connecting multiple external signers to the validator client. This might become interesting if there are more DVT solutions like Diva, or just in general, if a user wants to join multiple Diva clusters without running multiple Lodestar instances.

Solution description

Introduce --externalSigner.urls which allows to specify multiple external signing servers. The design of our validator store already supports this, just need to tweak the CLI a bit, and how public keys are fetched.

Additional context

No response