* De-share the Handler struct in core API
An upcoming PR adds a handler that only applies on one of these paths.
Having fields that don't work seems bad.
This never should have been shared. Lifecycle hooks are like a "write"
while probes are more like a "read". HTTPGet and TCPSocket don't really
make sense as lifecycle hooks (but I can't take that back). When we add
gRPC, it is EXPLICITLY a health check (defined by gRPC) not an arbitrary
RPC - so a probe makes sense but a hook does not.
In the future I can also see adding lifecycle hooks that don't make
sense as probes. E.g. 'sleep' is a common lifecycle request. The only
option is `exec`, which requires having a sleep binary in your image.
* Run update scripts
For containers that don't have bash, we should support env substitution
like we do on command and args. However, without major refactoring
valueFrom is not supportable from inside the prober. For now, implement
substitution based on hardcoded env and leave TODOs for future work.
