[csolution-rpc] Add DiscoverLayers method

[brondani]

Fixes

• Relates to [csolution rpc] Add methods for discovering layers and listing missing packs cmsis-toolbox#419

Changes

• The response of the DiscoverLayers method has a similar structure to the special configurations node written in *.cbuild-idx.yml for the same purposes, but it has a simplification: it is not grouped over several target-types because this method requires an active target-set which only allows a single target-type.

Checklist

• 🤖 This change is covered by unit tests (if applicable).
• 🤹 Manual testing has been performed (if necessary).
• 🛡️ Security impacts have been considered (if relevant).
• 📖 Documentation updates are complete (if required).
• 🧠 Third-party dependencies and TPIP updated (if required).