Ensure coordinator node and cross validation nodes are different

### Describe the problem you discovered

When a replication node verify the validation stamps and cross validation stamps signature, it does not ensure the coordinator node and the cross validation nodes are different.
This can be a breach for a malicious node to send a self signed transaction to storage nodes while it should be signed through the distributed workflow with multiple nodes.

### Describe the solution you'd like

In the function `Transaction.valid_stamps_signature?` we should ensure the coordinator node is different from the cross validation nodes.
This check should be done only if the validation node list has at least 2 nodes

Start at Transaction.valid_stamps_signature? and trace how the coordinator and cross-validation node lists are represented. Verify behavior with fewer than two validation nodes and with two or more, where overlapping coordinator and cross-validation nodes should be rejected; the payload names no test file, so locate the relevant existing tests before running them.

Written by the indexing model from the issue text.