[Helper::Utils] Add method to be able to manually set SOFAPathPrefix to override getExecutablePath if SOFA_ROOT is not set

[epernod]

I need this method when I'm shipping SOFA dll which are used froma third party software.

Right now the main mechanism is to set SOFA_ROOT to explicitly define where is the build/install of SOFA with it's internal structure: bin/ data/ and even /etc/sofa/python.d/ which is used by PythonEnvironment to load site-packages
I can't rely on this mechanism as I can't ask all users to set a env variable (especially not IT users).

The current fallback solution if SOFA_ROOT is not set is to use the current executable path. This solution is not always relevant, for example, inside Unity3D, the path would be something like: C:/Program Files/Unity/Unity.exe which doesn't correspond to the current project path.

The method just allows to set a custom path. Then, if the path is set we don't call computeSofaPathPrefix() which is doing the normal mechanism:

const char* pathVar = getenv("SOFA_ROOT");
if (pathVar != nullptr && FileSystem::exists(pathVar))
{
    return FileSystem::convertBackSlashesToSlashes(pathVar);
}
else {
    const std::string exePath = Utils::getExecutablePath();
...

[with-all-tests]

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