feat: Support multiple DNS discovery trees

[jm-clius]

Problem

See status-im/infra-status#17. After discussion with @jakubgs it became clear that supporting multiple DNS discovery trees in nwaku could be very useful. This will make adding a list of nodes as easy as repeating the --dns-discovery-url: CLI option and allows for bootstrap nodes to maintained in separate lists, where necessary and without having to rely on the "linked-tree" concept that exists in EIP-1459.

Acceptance criteria

• repeated --dns-discovery-url: allowed, bootstrapping connection to each tree