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