Add tool to generate random XDR values

[tamirms]

Given an XDR data type (e.g. LedgerEntryChange) we would like to generate a set of XDR values which cover all possible structures of the data type (e.g. given an xdr union we want to make sure all branches of a union are represented in the set). The fields within the XDR values should be set to random values.

The primary motivation behind building such a tool is that we could use it to generate a corpus for fuzz testing. We could also potentially integrate this tool into a property testing framework. This tool would be particularly helpful when working on protocol upgrades which require us to update the Stellar XDR definitions (e.g. #3430)