The newly introduced hubble-proxy component needs to connect to a hubble
server via a unix domain socket in order to query the API for initial
peer discovery. This commit updates cilium-agent such that when hubble
is enabled, it automatically listens on a unix domain socket.

In order to easily enable or disable the hubble server, a new switch is
also added. Note that it using unix domain sockets as listen address is
no longer enforced and a warning is emitted instead. In the future,
mutual TLS will be implemented to secure connections but for the time
being, allowing to listen on a TCP port will allow development of the
multi-node feature for Hubble.

This commit also updates some logging messages to the error level rather
than warning when the hubble component cannot be started.

Note that some variables (api => restAPI) had to be renamed to avoid a
name conflict when importing the api package but there are no functional
changes there.

Signed-off-by: Robin Hahling <robin.hahling@gw-computing.net>

This commit adds package pkg/hubbel/proxy with an implementation of
hubble's ObserverServer interface. For the time being, the
implementation simply connects to the locally running hubble instance
via unix domain sockets and forward requests there. In the future, the
proxy API will be extended and decoupled from hubble's own API.

Signed-off-by: Robin Hahling <robin.hahling@gw-computing.net>

The newly introduce 'serve' command runs a hubble-proxy server that, for
the time being, forwards requests to the local hubble instance connected
to via unix domain socket.

Signed-off-by: Robin Hahling <robin.hahling@gw-computing.net>

This commit brings no functional changes.

Signed-off-by: Robin Hahling <robin.hahling@gw-computing.net>