ucode: add ucode interpreter plugin

ucode: add ucode interpreter plugin

The rpcd ucode plugin allows utilizing ucode scripts to register ubus
objects and to implement the objects method callbacks.

Upon startup, rpcd will compile and execute each ucode script in
`$INSTALL_PREFIX/share/ucode/` and register ubus proxy objects and
methods definitions according to the signature returned by the script.

Refer to examples/ucode/example-plugin.uc for details of the signature
format.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>