CLI: support wasm_c_api backends together with native ones

[xdoardo]

We recently introduced a new mechanism to interact with backends implementing the wasm_c_api spec. Currently, we can't have a single CLI binary that supports both wasm_c_api backends and "native" ones (LLVM, cranelift and singlepass), due to the different features they have (wasm_c_api vs sys). We should find a way to allow heterogeneous backends to be embedded in the same binary; a proposal is that of using a plugin-based architecture for the CLI.