admin: Introduce virAdmClientGetInfo API

admin: Introduce virAdmClientGetInfo API

Expose a public API to retrieve some identity and connection information about
a client connected to the specified server on daemon. The identity info
retrieved is mostly connection transport dependent, i.e. there won't be any
socket address returned for a local (UNIX socket) connection, while on the
other hand, when connected through TLS or unencrypted TCP, obviously no UNIX
process identification will be present in the returned data. All supported
values that can be returned in typed params are exposed and documented in
include/libvirt/libvirt-admin.h

Signed-off-by: Erik Skultety <eskultet@redhat.com>