Make the node auto-detect its registry_address

[Geod24]

Currently, we have the following configuration item:

agora/source/agora/node/Config.d

Lines 329 to 330 in 8ed2d6d

	// Registry address
	public @Optional string registry_address;

As part of the validator config. However, there are a few problems with this:

• Most nodes will end up needing the registry, not just validators, hence it's in the wrong section;
• It is currently optional without a default, but it should have a sensible default;
• A sensible default depends on realm: hence, it could be inferred from realm;
• (Minor) the name is a bit too verbose;

What we should do is look up the NS records for the realm so that we get the authoritative registries for the realm we want to use. However, there might be a case for overriding the default registry (to avoid a SPOF, reduce load, or improve anonymity), hence we should have the ability to do so. For this, I suggest to turn the single validator.registry_address into node.registries, an array of Address with fuzzy parsing (as in, default to dns:// for scheme if there isn't any).