lineapy.save to support multiple variables/side-effects

[yifanwu]

Note: This is lifted out of #447 since the solution is different.

What: when lifting workflows out of a notebook/script, people might want to capture multiple side-effects (or even variables) because they conceptually group them in the same job.

Why: @marov's use case for exporting both to_csv and to_sql.

Possible Approaches:

• save to accept a set of variables/side-effects, so here it would be .save([lineapy.db, lineapy.file_system], "external writes").
• create a new abstraction similar to lineapy.db and lineapy.file_system but for all external side effects.