Install silkworm_core as a library

It would be nice if there were a CMake option to install the `silkworm_core` package as a library (e.g., SILKWORM_CORE_INSTALL). I tried making that change locally, but I couldn't find a way to have the `evmone` and `silkpre` targets installed without touching their own CMake files.

There must be an easier way to add the option for `silkworm_core` to be installed locally that doesn't involve making changes to the other targets. I believe this would be helpful for C++ developers creating programs that interact with EVM chains. This could start filling in the gaps from the deprecated [aleth](https://github.com/ethereum/aleth) and also serve as the basis for something like what Rust devs have available in [ethers-rs](https://github.com/gakonst/ethers-rs).