Query backend versions

[franzpoeschel]

Is your feature request related to a problem? If so, please describe.

Downstream applications might need to guard against bugs in e.g. ADIOS2 or HDF5. This is currently difficult, since it requires including backend-specific headers.

Describe the solution you'd like
Since backend versions can be changed in openPMD-api's design without an ABI change, this should ideally be runtime call.

We already have the following in version.hpp:

/** Return the feature variants of the openPMD-api library (run-time)
 *
 * @return std::map< std::string, bool > with variants such as backends
 */
std::map<std::string, bool> getVariants();

This cannot be extended to include version information without breaking API, but an additional call could help:

struct backend_information
{
     std::string version;
};

std::map<std::string, backend_information> getVariantsDetailed();

Describe alternatives you've considered

1. Using #define macros: Would require including backend headers into public headers, which goes against our project structure and would make the ABI of user code depend on the available backends. Currently, installing a new version of e.g. ADIOS2 only requires relinking downstream applications.
2. Fetching this information manually in downstream applications: Requires interacting with the backend libraries instead of going via openPMD-api.

Additional context
Required for implementing the open TODO in ComputationalRadiationPhysics/picongpu#4759

[ax3l]

That is great. We should even expose the version we compiled against and, if available, also the version we found at runtime. Let's start with compile-time versions we found though, we might not find runtime APIs for every dependency (we could check and report where we would like some).