pgsql: Move recovery injector astreamer to a separate header file.

Move recovery injector astreamer to a separate header file.

Unlike the rest of the astreamer (formerly bbstreamer) infrastructure
which is reusable by other tools, astreamer_inject.c seems extremely
specific to pg_basebackup. Hence, move the corresponding declarations
to a separate header file, so that we can move the rest of the code
without moving this.

Amul Sul, reviewed by Sravan Kumar and by me.

Discussion: http://postgr.es/m/CAAJ_b94StvLWrc_p4q-f7n3OPfr6GhL8_XuAg2aAaYZp1tF-nw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/53b2c921a0f9b56465ab65165c1909f9616ffa98

Modified Files
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src/bin/pg_basebackup/astreamer.h | 6 ------
src/bin/pg_basebackup/astreamer_inject.c | 2 +-
src/bin/pg_basebackup/astreamer_inject.h | 24 ++++++++++++++++++++++++
src/bin/pg_basebackup/pg_basebackup.c | 2 +-
4 files changed, 26 insertions(+), 8 deletions(-)