Add support for building your own *.csproj files

[hoffmann-stefan]

upstream PR for this internal one: schiller-de#1

New variants of add_dotnet_*

This PR adds the posibility to use your own *.csproj files instead of generating one with cmake.
This makes it easier to use advanced *.csproj features needed for ASP.NET as the developer can directly manipulate the .csproj file.

This is how the new commands corespond to the existing ones:

add_dotnet_executable -> add_dotnet_executable_project
add_dotnet_library -> add_dotnet_library_project
add_dotnet_test -> add_dotnet_test_project

Example:

...
set(_assemblies_dep_dlls
    ${rcldotnet_common_ASSEMBLIES_DLL}
    ${rcldotnet_ASSEMBLIES_DLL}
    ${std_msgs_ASSEMBLIES_DLL}
    ${std_srvs_ASSEMBLIES_DLL}
)

add_dotnet_executable_project(RosBlazorExample
  PROJ
  src/RosBlazorExample/RosBlazorExample.csproj
  INCLUDE_DLLS
  ${_assemblies_dep_dlls}
)

install_dotnet(RosBlazorExample
  CD_TO_EXECUTABLE
  ENTRY_POINT_NAME
  ros_blazor_example
  DESTINATION
  lib/${PROJECT_NAME}/dotnet
)
...

RosBlazorExample.csproj needs to import a generated file for this to work:

  <Import Project="obj/CMake.g.props"/>