Allow newer C++ standard flags when building an application

[alexcamposruiz]

Description

The argument LANG of the function connextdds_add_application should allow C++14, C++17, C++20, C++23, etc. It's currently restricted to C++98 or C++11.

While this restriction makes sense in the code generation (connextdds_call_codegen), it should be possible to compile the code with newer C++ standards.

Suggested solutions

Allow for ex:

connextdds_add_application(
   ...
   LANG "C++20"
   ...
)