Endpoint discovery and filtering

#24 (closed) deals with adding custom endpoints.

In https://forum.duniter.org/t/liste-des-endpoints/12347 we discussed how to retrieve a list of endpoints automatically in the clients. While the server part is not finalized yet (only a hacky temporary solution using onchain remarks and indexer has been set up), the client part can be implemented from now on.

We have to implement some kind of network discovery and filtering since it's not part of the polkadot ecosystem which relies on trusted endpoints: https://wiki.polkadot.network/docs/maintain-endpoints. These are the steps we can implement:

1. local format check of the URL (wss protocol, domain...)
2. genesis hash must match the selected network (to avoid interacting with test network)
3. synchronisation state should be coherent (with an estimation of the acceptable range for latest head)

Once the list is filtered, we can use endpoints to establish connections with the blockchain.

The same can be done for indexer endpoints.

added priority:severe label

changed milestone to %2.0.0

added ToDo label

We now have a shared repo https://git.duniter.org/nodes/networks to pick endpoints from.

This does not spare from implementing filtering but at least provides a list where most endpoints should be online and well configured.

This list can be fetched at startup to see if other endpoints were added.

created branch 35-endpoint-discovery-and-filtering to address this issue

mentioned in merge request !34

removed ToDo label

added Doing label