Provide a list of federated instances upon first contact to better facilitate new instances #36766
Description
Pitch
Federation is slow. Not just kinda slow, but really slow. A new instance can go several days before seeing some instances, and there's a small chance that it will never see some instances.
This is fixable. If each Mastodon instance maintains a list of every other instance that it knows about, it can send this list to a new instance upon first contact. This gives the new instance a head start in discovering other communities.
It would be, by design, separated from moderation blocklists, since these can often reflect personal opinion as opposed to actual risk - and for belt and braces, implementing a new blocklist CSV could also trigger the option to purge any related content upon user request - rather than going through each individual domain.
The implementation doesn't need to be overly complex - a short CSV or JSON containing a list of instance domain names and their instance names would suffice. This is a minute amount of data to be sending.
An alternative implementation is to permit a one-time poll of these data from the administration dashboard - instances that find others quickly might not even need the function, thus traffic could be reduced even further.
Motivation
Any new instance must wait an ostensibly significant amount of time before being functional as a part of the greater fediverse network. Mine in particular has now been up for over 48hrs, and whilst I have thirty pages of other instances, I still cannot find the majority of the instances I require to properly migrate my account.
If, however, I could poll my old instance for a list of instance identities, I could shorten this time to perhaps less than ten or fifteen minutes. Waiting for such a long time is a horrendous hurdle, and this approach eliminates it almost entirely.