Support for SEP 10 challenge transaction

[ire-and-curses]

SEP 10 describes a specific form for the "challenge" transaction used for authentication. Providing a helper at the SDK level makes sense to save users from reimplementing the spec.

Here are example implementations in Go and JS.

[jillesvangurp]

We will rely on whatever is implemented for this in the Java SDK. I have an open pr for refactoring the federation client in that library: lightsail-network/java-stellar-sdk#233

Also may consider this later when implementing the new Kotlin client as discussed in #2