wip: plugins poc draft

Summary

• wip: plugins poc draft

Here are the 2 new crates:

• sdk/picodata-sdk - sdk for plugins creation
• picodata-types - initially for types needed both in sdk and in picodata-server

sdk/plugin-example - is a minimal example to test the poc

Additionally:

• removed app.rs
• added the common workspace

Ensure that

• New code is covered by tests
• API is documented
• Changelog is up to date
• (if Lua API changed) Lua API version is bumped in luamod.rs
• (if necessary) A follow-up doc issue is created in picodata/docs and linked here