Support plan-level AdvancedExtension in the text format

[wackywendell]

substrait-explain should support the top-level Plan.advanced_extensions field when parsing and formatting the text format.

This is related to #160, which adds === Header and Plan.version support. Version support should land first so the document-level parser/textifier path has a clear place for top-level plan fields. Plan-level advanced extensions should be a separate issue because their best placement is a distinct syntax decision.

The existing text format already supports relation-level advanced extensions with + Enh: and + Opt: addenda attached below a relation. For the top-level Plan.advanced_extensions field, one option is to place them directly under === Plan, before root relations:

=== Plan
+ Enh:SomePlanEnhancement[...]
+ Opt:SomePlanOptimization[...]
Read[table => col:i32]

That keeps + Enh: / + Opt: attached to the same conceptual area as relation addenda, while using top-level indentation to distinguish plan-level advanced extensions from relation-level ones.

Alternative Placements

One alternative is to put them in === Header, because Plan.advanced_extensions is a top-level Plan field:

=== Header
Version: 0.64.0
Advanced Extensions:
  + Enh:SomePlanEnhancement[...]
  + Opt:SomePlanOptimization[...]

Another alternative is to put them in === Extensions, because the payloads are extension-shaped and registry-backed:

=== Extensions
URNs:
  @  1: extension:io.substrait:functions_arithmetic
Advanced Extensions:
  + Enh:SomePlanEnhancement[...]
  + Opt:SomePlanOptimization[...]

I prefer putting them directly under === Plan because they modify the plan rather than declaring extension anchors or describing decode compatibility, but either alternate is workable if we want all top-level fields in one place.

Acceptance Criteria

• Parse plan-level + Enh: / + Opt: lines into Plan.advanced_extensions.
• Format Plan.advanced_extensions back into text.
• Preserve relation-level advanced extension behavior.
• Add parser/textifier roundtrip coverage.
• Update GRAMMAR.md and doc tests as needed.