Support SubQueries interpretation as expressions

Summarize the changes

Support SubQueries as expressions. Previously they were supported only as a part of IN or EXISTS operators. After this patch they can be used as a scalar expressions too.

Closes:

• #685 (closed)
• #683 (closed)
• #601 (closed)
• #454 (closed)
• #154 (closed)

Ideologically unrelated but closes too:

• #704

Picodata follow-up: picodata!1229 (closed)

Ensure that

• New code is covered by unit and integration tests.
• Related issues would be automatically closed with gitlab's closing pattern (Closes #1, #2).
• Public modules are documented (check the rendered version with cargo doc --open).
• (if PEST grammar is changed) EBNF grammar reflects these changes (check the result with railroad diagram generator.

Next steps

• Cherry-pick to: none
• Update sbroad submodule in picodata/picodata.