plpgsql: implement FOREACH loops

This is a tracking issue for implementing the PL/pgSQL FOREACH statement, Postgres docs [here](https://www.postgresql.org/docs/current/plpgsql-control-structures.html#PLPGSQL-FOREACH-ARRAY). FOREACH is used to iterate through the elements of it array argument. Postgres also supports a SLICE option that allows iterating through slices of a multi-dimensional array. Since we don't yet support multi-dimensional arrays, we can probably ignore SLICE for now. Ex:
```
FOREACH x SLICE 1 IN ARRAY $1 LOOP ... END LOOP;
```

Jira issue: CRDB-60074