Transactions - support ExtCustomMetadataCollection

This extension allows users to specify a custom collection in which transactional metadata documents (ATRs and client records) will be stored. Refer to CBD-4874 for details. The majority of this implementation is in the underlying C++ logic, but wrappers will need to expose the API and forward to C++.

I think Python has this already supported at the global config level, but it also needs to be supported at the per-transaction level - which is going to require C++ support first.