Implement nested repeat directive

[johngrimes]

Currently the repeat directive only works at the top level.

This change will expand the implementation so that it can be placed at arbitrary levels of nesting.

[johngrimes]

@piotrszul Create a PR on https://github.com/FHIR/sql-on-fhir-v2 to add new tests to the test suite, as discussed.

You can use a branch on https://github.com/aehrc/sql-on-fhir-v2 for this.

[piotrszul]

Clarifying the intent of this change, since the earlier design notes left some ambiguity:

The goal is to support repeat with no positional constraints beyond what the SQL on FHIR v2 spec itself requires. The spec's only structural rule on the iteration modifiers is per-select-node:

(forEach.exists().toInteger() + forEachOrNull.exists().toInteger() + repeat.exists().toInteger()) <= 1

— at most one of forEach, forEachOrNull, or repeat on any given select node. There is no rule about depth, siblings, or which other operators may surround a select node that carries repeat. repeat is a modifier of a select node, not a distinct node type, so wherever a select is structurally legal, a select-with-repeat is structurally legal.

Concretely, the implementation must produce correct SQL for any spec-valid tree, including (non-exhaustive):

• repeat nested inside forEach, forEachOrNull, another repeat, or a unionAll branch
• forEach, forEachOrNull, another repeat, or unionAll nested inside a repeat
• multiple repeat-bearing siblings under any parent
• arbitrary depth of nesting and arbitrary combinations of the above

Scope of this issue:

1. Implement repeat under that requirement.
2. Add new test cases to the SQL on FHIR v2 test suite covering nested and sibling repeat combinations.