Make stream_prepare an optional callback

Hi Hackers,

As part of commit 0aa8a0
<https://github.com/postgres/postgres/commit/0aa8a01d04c8fe200b7a106878eebc3d0af9105c>
,
new plugin methods (callbacks) were defined for enabling two_phase commits.
5 callbacks were required:
* begin_prepare
* prepare
* commit_prepared
* rollback_prepared
* stream_prepare

and 1 callback was optional:
* filter_prepare

I don't think stream_prepare should be made a required callback for
enabling two_phase commits. stream_prepare callback is required when a
logical replication slot is configured both for streaming in-progress
transactions and two_phase commits. Plugins can and should be allowed to
disallow this combination of allowing both streaming and two_phase at the
same time. In which case, stream_prepare should be an optional callback.
Attaching a patch that makes this change. Let me know if you have any
comments.

regards,
Ajin Cherian
Fujitsu Australia.