Dynamic queries: add support for predicates in WHERE clauses

[jschaf]

Use case is to enable queries with dynamic expressions for WHERE clauses, like:

SELECT *
FROM foo
WHERE pggen.predicate('filter_expr')

Sketch:

• Generated Go code takes a FilterExpr function that returns (string, error), maybe implementing a generic interface so it's easy to wrap existing Go SQL codegen.
• If we implement an interface, we should move it to a different library so folks don't have to depend on the relatively heavyweight dependencies of pggen, namely Docker.
• pggen calls FilterExpr and injects code as-is into the query.