Add a check for interrupts in ginInsert

Hello,
Please find attached a small patch that adds the ability to check for
interrupts during the insert into the GIN index. Currently, ginInsert
can generate a large number of entries for a given input tuple from
extractEntries. In the case of Json/Jsonb for instance, the standard
opclasses may generate 1 term per path & value (which may be large).
During index build, the build process does check for interrupts in
between the inserts of each entry into the index (since there may be a
large number of entries to insert). However, the same loop during
insert was not checking for interrupts. This patch adds an interrupt
check to allow for early termination if the overall request is
abandoned. I believe this should be a safe point for checking for
interrupts since we should not be in the middle of modifying the index
tree in between entries.

Thanks and Regards,
Vinod Sridharan
[Microsoft]