'graph-download-snapshot' to support various sources

[maxim-uvarov]

For the purpose of decentralization, it would be beneficial to have the ability to download graph snapshots using various indexers and data providers, not limited to just graphkeeper. This would enable cross-checking of snapshots and enhance failure tolerance.

Additionally, a merging algorithm is required to combine data from different providers.

A user should be able to subscribe for several data providers and download data from all of them in parallel.