Return CAS value inside Transaction context calls (eg. CoreTransactionGetResult)

The `cas` value is not available in the returned objects inside transactions.

We had to use java reflection to locate and read the cas value.
 We need the cas value in our use cases from an insert/replace calls.

(Use case: We return the cas value in our API for later updates with optimistic locking)

Demonstration code provided:

https://github.com/spirostz/couchbase-transactions/blob/main/src/test/java/org/spiros/CouchbaseTransactionsTest.java

See: "Problem 2 non-proper solution" in the code below