services/horizon/cmd: spike, options on how to obtain archive server list

[sreuland]

What problem does your feature solve?

NETWORK=testnet|pubnet usage will trigger default hardcoded archive url configuration on HISTORY_ARCHIVE_URLS to public stellar archive hosts, which doesn't promote decentralization in practice. HISTORY_ARCHIVE_URLS can still be set by users to override, but is not required.

What would you like to see?

when using NETWORK parameter, the configuration of servers for HISTORY_ARCHIVE_URLS should promote decentralization, how to accomplish this needs to be explored.

Research the options to obtain a decentralized list of archive server hosts and provide a write-up on a separate doc or here on ticket. Include a summarized table which compares/contrasts top 2 or 3 options.

for starters, a few options:

• don't set any hardcoded archive servers by default with NETWORK usage, thereby requires HISTORY_ARCHIVE_URLS to be defined by users when they are running an ingesting instance of horizon.
• hardcode a blended list from the ecosystem for archive servers into HISTORY_ARCHIVE_URLS when using NETWORK, this requires maintenance over time.
• use a service locator pattern for horizon to obtain a list of archive servers dynamically by querying a separate service, however, this results in a central source for the lookup though, similar to DNS.
• derive archive urls from the quorum set defined in captive core config of the tier 1 validators.

What alternatives are there?

[sreuland]

discussion on #4949 is the source context, it explains the need for focus on decentralized approach the archive server urls.

[sreuland]

consider deriving archive urls from captive core config quorum set for tier 1 validators.