GUCs to control abbreviated sort keys

The attached patch adds GUCs to control the use of the abbreviated keys
optimization when sorting. Also, I changed the TRUST_STRXFRM from a
#define into a GUC.

One reason for these GUCs is to make it easier to diagnose any issues
that come up with my collation work. Another is that I observed cases
with ICU where the abbreviated keys optimization resulted in a ~8-10%
regression, and it would be good to have some basic control over it.

I made them developer options because they are more about diagnosing
and I don't expect users to change these in production. If the issues
with abbreviated keys get more complex (where maybe we need to consider
costing each provider?), we can make it more user-facing.

This is fairly simple, so I plan to commit soon.

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Jeff Davis
PostgreSQL Contributor Team - AWS