MINOR: proxy: implement "show defaults"

MINOR: proxy: implement "show defaults"

Implement a new command "show defaults" whose purpose is to list the
existing named defaults section. This may be needed when adding a new
backend via the CLI.

For now, only the names of the defaults instances are listed. In the
future, it could be useful to have at least some details about their
configuration.

Command does not take any argument and is implemented via a single
io_handler. No need for a watcher as iteration is performed over
defaults instances which cannot be removed at runtime.