Feat/sdk poc

src/lib.rs now contains the public api used by user defined plugins

an example plugin can be found in roles/toy currently the plugin must define these things:

• a static #[no_mangle] variable called ROLE_DECLARATION of type picodata::RoleDecl
• an unsafe extern "C" fn register which will be called by picodata when loading the plugin
• a struct implementing the picodata::Role trait, which must be passed to the picodata::RoleRegistrar::register function

This list seems kinda bloated and redundant, maybe we should trim it down a bit?

to test run

cargo build --all # so that the libtoy.so plugin is compiled
cargo run -- run --roles toy # the libtoy.so will be looked for in the target/debug directory