Add an option to disable CMake Unity builds

[amatria]

Pull Request Prelude

• [x] I have followed proper The Forgotten Server code styling.
• [x] I have read and understood the contribution guidelines before making this PR.
• [x] I am aware that this PR may be closed if the above-mentioned criteria are not fulfilled.

Changes Proposed

Add a CMake option to disable Unity builds. clangd, for instance, does not support unity builds, so we'd rather add this convenient switch to turn them off.

Issues addressed: None