[CALCITE-5280] Add geometry aggregate functions (ST_Accum, ST_Collect, and ST_Union)

[bchapuis]

Here, the main point of caution is the creation of the operator table in SqlLibraryOperatorTableFactory.

[julianhyde]

The large number of new imports is an indication that the code to define the aggregate functions should be somewhere else.

[bchapuis]

I agree, this code should have been placed in SqlOperatorTables.createSpatial. However, as it does not address the number of imports, I looked at the SqlStdOperatorTable and wondered if we should create a SqlSpatialOperatorTable. I will probably dig into this a bit, but please stop me if it is a bad idea.

[bchapuis]

@julianhyde I moved this part of the code to a new class called SqlSpatialTypeOperatorTable, which implements the SqlOperatorTable interface. I also experimented with the ReflectiveSqlOperatorTable, but I believe that using this approach would require declaring properties of type SqlUserDefinedAggFunction for each AggregateFunction. What do you think about continuing down this path?