providing local cache support for service discovery when registry is unavailable #13856

[ghostly72]

Currently, Dubbo’s registry layer writes provider information to a local cache file, but it does not read that cache when the registry center becomes unavailable. So, consumers fail to discover providers during temporary registry outages, even though the information already exists locally. This PR introduces a fallback mechanism so that consumers can still find providers using the last known data in the local cache.

Added some code in org.apache.dubbo.registry.support.AbstractRegistry to: Load provider URLs from the local cache file when the registry cannot be reached. Immediately notify registered listeners with cached data so consumers remain functional.

The fallback is triggered only when the registry is unavailable or not yet ready.