ci: add build that runs xdrgen on stellar/go and prints out diff

[leighmcculloch]

What

Add build that runs xdrgen on stellar/go and prints out diff.

Why

In PRs where we've made major changes to the Go implementation, viewing a diff of the generated code has been invaluable. While xdrgen is intended to work with any XDR, the Stellar XDR is the major consumer and use case and it would be beneficial to have that visible in builds.