pgsql: Allow extensions to add new backup targets.

Allow extensions to add new backup targets.

Commit 3500ccc39b0dadd1068a03938e4b8ff562587ccc allowed for base backup
targets, meaning that we could do something with the backup other than
send it to the client, but all of those targets had to be baked in to
the core code. This commit makes it possible for extensions to define
additional backup targets.

Patch by me, reviewed by Abhijit Menon-Sen.

Discussion: http://postgr.es/m/CA+TgmoaqvdT-u3nt+_kkZ7bgDAyqDB0i-+XOMmr5JN2Rd37hxw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e4ba69f3f4a1b997aa493cc02e563a91c0f35b87

Modified Files
--------------
src/backend/replication/Makefile            |   1 +
src/backend/replication/Makefile.orig       |  49 ++++++
src/backend/replication/basebackup.c        |  82 ++++------
src/backend/replication/basebackup_target.c | 238 ++++++++++++++++++++++++++++
src/include/replication/basebackup_target.h |  66 ++++++++
5 files changed, 381 insertions(+), 55 deletions(-)