Add an additional C API example "aggregates_string" which demonstrates running min/max on string dimension and emitting coordinates from the same query

[rroelke]

An example like this would have been a useful reference when building the query aggregate API in tiledb-rs (and in fact it shows that we did a few things wrong over there).

So, we combine the quickstart_sparse_string.c and aggregates.c examples into a new example which demonstrates the above.

I have also added tiledb_examples.h with some functions/macros to do error checking. Whenever I have played with one of the examples to make a bug reproducer or learn a bit I am adding a bunch of error checks to see if I am doing anything wrong. Error checking should be a standard part of the examples and this header provides a fairly non-intrusive way to do it.

---

TYPE: IMPROVEMENT DESC: Add aggregates_string.c example and tiledb_examples.h for common example code

[KiterLuc]

+1 on adding the matching CPP API example.

[rroelke]

I have added a corresponding C++ example