JSONSchema: add idmap parser and storagepair format

generalized from the start to support extension to bridges or other
entities as well.

this gets us incremental support for the CLI, e.g.:

--targetstorage foo:bar --targetstorage bar:baz --targetstorage foo

creates a mapping of

foo=>bar
bar=>baz

with a default of foo

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>