[Patch] Implement TODOs for index operator strategy number validation

Hi,

This patch implements TODOs in the BRIN, GIN, GIST and SP-GIST index code.
Operator class validation is modified to add strategy number bounds
checking specific to each index type.
1) For BRIN and SP-GIST, the maximum strategy number from the common
RTStrategy list is used as the upper limit.
2) For SP-GIST, the number of GIS object types and the strategy numbers per
type are used to calculate the maximum strategy number.
3) As far as I can understand from the code, GIN may have custom
strategies, so the existing limit of 63 is retained.

I have tested this patch using 'make check' and 'make worldcheck'. All the
tests pass.

This is my first contribution, so any input would be appreciated.

Thanks & regards,
Priyanka