[Impl] Vald Index Exporter / Importer

[vankichi]

Is your feature request related to a problem? Please describe.:

Currently, each Agent individually backs up index information to a file.
However, if the number of Agents decreases, there's a risk of both index and file loss.
While replication can mitigate this risk, it's still a potential failure scenario.

Describe the solution you'd like:

Back up index data across a cluster into a single file that can be used to restore to a Vald cluster