feat: introduce IF EXISTS and IF NOT EXISTS options

Summarize the changes

• feat: introduce IF EXISTS and IF NOT EXISTS options

Picodata: picodata!1291 (merged)

Closes #662 (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.
• (if EBNF grammar is changed) create a follow-up issue in picodata/docs -- docs#371 (closed).

Next steps

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