Make all third-party dependencies optional and downloadable via CMake

[dtbpkmte]

Is your feature request related to a problem? Please describe.
The users have to download and install each dependency (e.g. sockcanpp, serial) before building mumei, which is inconvenient.

Describe the solution you'd like
Add a CMake rule to download and install dependencies if necessary, like the way we do with gtest.

Describe alternatives you've considered
N/A

Additional context
N/A