relocating the server's backup manifest code

Hi,

The backup manifest patch added a bunch of new code to
src/backend/replication/basebackup.c, and while lamenting the
complexity of that source file yesterday, I suddenly realized that I'd
unwittingly contributed to making that problem worse, and that it
would be quite easy to move the code added by that patch into a
separate file. Attached is a patch to do that.

Despite the fact that we are after feature freeze, I think it would be
a good idea to commit this to PG 13. It could be saved for PG 14, but
that will make future back-patching substantially harder. Also, a
patch that's just moving code is pretty low-risk.

Thoughts?

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company