Description: Taking advantage of the new feature introduced in [#12035](https://github.com/envoyproxy/envoy/pull/12035), which introduced quick visibility for init managers to check unready targets, this pull request adds protobuf message for unready targets and enables admin to dump configs of unready targets. An example of config dump for listeners’ unready targets is given in this pull request.
Introduce ```InitDumpHandler``` with ```handlerInitDump``` method to help dump information of unready targets.
Add ```dumpUnreadyTargets``` function for ```init::manager```.
Risk Level: Low
Docs Changes: protodoc
Release Notes: Added
Signed-off-by: pingsun <pingsun@google.com>
Mirrored from https://github.com/envoyproxy/envoy @ 8aef76370877c66b09f7791f0577ca83aad7d608
data-plane-api(CircleCI)