injection_points: Move some structs to new header injection_points.h

injection_points: Move some structs to new header injection_points.h

This commit moves the definitions of InjectionPointConditionType and
InjectionPointCondition into a new header local to the test module
injection_points.h, so as these can be shared across more files in the
module.  A patch for a bug fix is under discussion, whose proposed test
will benefit from this refactoring.

Backpatch down to where the module exists, as this should be useful for
future bug fixes, even cases unrelated to the thread where this change
has been discussed.

Author: Andrey Borodin <x4mmm@yandex-team.ru>
Author: Vlad Lesin <vladlesin@gmail.com>
Discussion: https://postgr.es/m/d2983796-2603-41b7-a66e-fc8489ddb954@gmail.com
Backpatch-through: 17