Export the relational and sampling operators as Python

### Task Summary

Sixteen operators that rearrange rows, rather than read what is inside one, still have no standalone form: the three joins, the three set operations, Aggregate, the sort family, Split, the two samplers, and If, Dummy and Sleep. A workflow built out of them exports a script of commented placeholders.

Each implements `generateStandaloneCode`, returning the pandas that does what its executor does.

The samplers need more than that. A sampler decides per row whether to keep it, so which rows survive is fixed by the exact sequence `java.util.Random` produces. Seeding Python's own generator, or numpy's, with the same seed selects a different set, and the script would then report a different sample than the workflow it came from. The generator is transcribed instead, in a helper the three operators that draw from one share.

Step 11 of 27 in #8325.