Improvements for sygus query generation (#8224)

Improvements for sygus query generation (#8224)

Makes the two sygus query generators have a common base class, which makes it so that they both can dump queries.

Also fixes the expression miner to use user-level sygus terms, otherwise we can make queries using invalid internal terms, e.g. SEQ_NTH_OOB skolem.

The majority of the diff in this PR is moving query_generator to query_generator_sample_sat, and adding a basic version of query generation.