add capability to dump condDBv2 payloads, tags, GTs in XML format

Add capability to dump condDBv2 payloads, tags, GTs in XML format. This is the first version with dynamically compiled code. Use "conddb dump --help" to see details.

Also, as an example of the usage in a C++ test, a small example reading an xml file from a dump is implemented in CondFormats/RunInfo/tests/fromXML.cpp.