Bloom filter collator npe fix

Fixed up.
The operator assembler assumed that the loader stream could only be an index scan, in this case it was a project on top of the index scan.  Now the BloomFilter depends on the rowtype of the input stream instead of an exact operator
A PostgresServerBloomFilterIT has been added to test this
Instead of manually worrying about TString, etc it looks like the `findCollators()` helper method might already do what we want?