Provide shared parts directories to instances via ${slap-connection:shared-parts}.

Shared parts paths are already available in software buildout during compilation via ${buildout:shared-part-list}. This means currently, Theia must be compiled with shared parts enabled in order to forward the shared parts paths to the embedded slaprunner. And binary cache (currently) only supports softwares compiled without shared parts. It is possible to install a Theia from binary cache (therefore compiled without shared parts) in a node where shared parts are available, but the slaprunner embedded in Theia will then not be aware of the node's shared parts paths.

This change will allow Theia instances installed from binary cache to access the node's shared parts.

We will then be able - if we wish - to e.g. regularly compile the latest erp5 SR on the node and let every Theia instance access the shared parts, speeding up compilation of erp5 in the embedded slaprunners. Just as an example.

This might not be something we want to do on every node for various reasons, but it'd be useful to have the ability. One usecase is for slapos courses, to speed up compilation of the tutorial SR by precompiling shared parts.