Aggregate walks through all matching rules (including nested ones) and appends them into a list:

```yaml
rule:
  semantics: aggregate
  match:
    - condition: "true"
      output: "'FOO'"
    - condition: "true"
      output: "'BAR'"

# Output: ['FOO','BAR']
```

Few noteworthy design decisions below. All examples assume all conditions matched:

1. For usability reasons, subrules under an aggregate ancestor will always have their **lists flattened**:

```YAML
name: aggregate_flat_flattening_example
rule:
  semantics: aggregate
  match:
    - rule:
        semantics: first_match
        match:
          - condition: "resource.is_admin == true"
            rule:
              semantics: aggregate
              match:
                - condition: "resource.has_standard_pii == true"
                  output: "'GDPR_STANDARD'"
                - condition: "resource.is_b2c == true"
                  output: "'EU_B2C_NOTICE'"
    - condition: "true"
      output: "'FALLBACK'"

# Output: ['GDPR_STANDARD', 'EU_B2C_NOTICE', 'FALLBACK']
```

2. Base case of an aggregate rule is an empty list. Nested conditional rules within an aggregate rule which outputs `optional.none()` are pruned (except in cases where policy output explicitly emits an `optional.none()`):

```YAML
name: optional_pruning_example
rule:
  semantics: aggregate
  match:
    - rule:
        semantics: first_match
        match:
          - condition: "1 == 2"
            output: "'EU_NOTICE'"
    - condition: "true"
      output: "'ALWAYS'"

# Output: ['ALWAYS']
```

```YAML
name: explicit_optional_none_example
rule:
  semantics: aggregate
  match:
    - rule:
        semantics: first_match
        match:
          - condition: "resource.is_b2c == true"
            output: "optional.none()"   # Explicitly authored by user

    - condition: "true"
      output: "optional.of('ALWAYS')"

# Output: [optional.none(), optional.of('ALWAYS')]
```

PiperOrigin-RevId: 913930387