Refactor getNetwork and getVersionInfo endpoint to fetch captive core version without making http request to core-client

[psheth9]

What problem does your feature solve?

Currently getNetowrk and getVersionInfo endpoint calls /info on core client, which in theory can semi-block the core thread. We need to refactor captive core implementation to expose captive core version so that soroban-rpc does not fetch it using http client. (i.e cache it at core process level)

More context: #132 (comment)

What would you like to see?

• Update captive_core_backend.go to expose ProtocolVersion instead of fetching from core client
• Add caching or update version value when core process starts/restarts
• Refactor getNetwork and getVersionInfo endpoint to fetch value from captive_core object instead of calling http client

What alternatives are there?

N/A