[Server] Discovery Cache

[chr-hertel]

The implementation for discovering capabilities for the registry is using the Finder and Reflection to parse folders and files - which creates some load that we don't want to have on every JSONRPC Request.

Therefore, we should bring back some kind of caching, based on one PSR interface, but not as part of the Registry, since it is a concern of the Discovery. Could be a decorator.

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Please announce yourself before working on this - so we can avoid duplicate effort.