[FEATURE] Support multi-line virtual tables for ease of reading

[eliot1480]

Summary

When it comes to representing large virtual tables with substrait-explain, it can sometimes get a little visually cluttered when everything has to be displayed on a single line. If we implemented a format to allow for every row of data to have it's own line, it'd be much easier to read.

Motivation

By having every row of data have it's own line instead of a single massive line, it makes things much easier to read

Proposed Solution

When converting from substrait-explain to protobuf, instead of parsing the entire massive table, we parse the rows line by line. And when we convert from protobuf to substrait-explain, we print the rows within the virtual tables line by line.

Additional Context

Going from this:

== Query
=== Plan
Root[id, name, age]
  Sort[($1, &DescNullsFirst) => $0, $1, $2]
    Read:Virtual[(1:i32, 'Alice':string?, 25:i32), (15:i32, 'Bob':string?, 30:i32), (5:i32, null:string?, 20:i32) => id:i32, name:string?, age:i32]

To this:

== Query
=== Plan
Root[id, name, age]
  Sort[($1, &DescNullsFirst) => $0, $1, $2]
    Read:Virtual[
    (1:i32, 'Alice':string?, 25:i32), 
    (15:i32, 'Bob':string?, 30:i32), 
    (5:i32, null:string?, 20:i32) 
    => id:i32, name:string?, age:i32]

Is much more readable.

There are several formats that can be done though:

== Query
=== Plan
Root[id, name, age]
  Sort[($1, &DescNullsFirst) => $0, $1, $2]
    Read:Virtual[
      (1:i32, 'Alice':string?, 25:i32), 
      (15:i32, 'Bob':string?, 30:i32), 
      (5:i32, null:string?, 20:i32) 
      => id:i32, name:string?, age:i32]

[wackywendell]

See also #97 and #105.

The proposed solution makes sense to me: additional lines enclosed by the [ is pretty reasonable; the alternative in #105 is also possible, but perhaps not preferred.

Some notes:

• Updating the parser for this might be a bit tricky; the current parser manually parses each line, then applies the Pest parser to that line; with multiple lines, that gets complicated - how does the manual parser know which lines to join? I like the as-is syntax, but we could perhaps make it easier for the parser without too much readability cost by requiring a prefix and indentation for additional lines (see below); that could be ..., or it could be - to make it shorter. (Or … which is really nice, but also UTF8 and not easy to type; right now, the whole plan is ASCII).

=== Plan
Root[id, name, age]
  Sort[($1, &DescNullsFirst) => $0, $1, $2]
    Read:Virtual[
      - (1:i32, 'Alice':string?, 25:i32), 
      - (15:i32, 'Bob':string?, 30:i32), 
      - (5:i32, null:string?, 20:i32) 
      - => id:i32, name:string?, age:i32]