Allow initializer_list to define PluginFrame header values
fix log output
Add base/Json.h: JSON encoder for e.g. sending data to plugins
Extend commit plugin to send the transaction list (Fate#316203)

Added 2 new plugin methods COMMITBEGIN/COMMITEND passing the
TransactionStepList as JSON in the body. libzypp(plugin:commit)
version bumped to 1.
changes 14.2.0 (2)