Add support for writing the MSRV to the clippy config

[foresterre]

• [ ] Implement writing the MSRV to the clippy config
• [ ] Consider whether to consilidate this option under the --write-msrv flag, or like writing a toolchain file (--write-toolchain-file), to make it a separate option.
• [ ] Consider when we do not want to write the MSRV to the clippy config, and whether we should add an option to disable it, enable it conditionally etc.

Related: #333