Move connect.h from fe_utils to src/include/common.

Noah Misch <noah@leadboat.com>

Commit: 41dae35532d40041297ee728eac1f4584af05570
Author: Noah Misch <noah@leadboat.com>
Date: 2020-08-10T16:22:58Z
Releases: 13.0
Move connect.h from fe_utils to src/include/common.

Any libpq client can use the header.  Clients include backend components
postgres_fdw, dblink, and logical replication apply worker.  Back-patch
to v10, because another fix needs this.  In released branches, just copy
the header and keep the original.

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