c++: allow overriding of visibility attributes

[asherikov]

When compiling a library that uses MCAP internally it is good practice to hide MCAP api's, but the visibility attribute defines do not allow this https://github.com/foxglove/mcap/blob/main/cpp/mcap/include/mcap/visibility.hpp. It would be nice to make them overridable in order to avoid hacking the code in applications.