Introduce Morselizer API, rewrite ParquetOpener to ParquetMorselizer (#21327)

~(Draft until I am sure I can use this API to make FileStream behave
better)~

## Which issue does this PR close?

- part of #20529
- Needed for #21351
- Broken out of #20820
- Closes #21427

## Rationale for this change

I can get 10% faster on many ClickBench queries by reordeirng files at
runtime. You can see it all working together here:
#21351

To do do, I need to rework the FileStream so that it can reorder
operations at runtime. Eventually that will include both CPU and IO.

This PR is a step in the direction by introducing the main Morsel API
and implementing it for Parquet. The next PR
(#21342) rewrites FileStream in
terms of the Morsel API

## What changes are included in this PR?

1. Add proposed `Morsel` API
2. Rewrite Parquet opener in terms of that API
3. Add an adapter layer (back to FileOpener, so I don't have to rewrite
FileStream in the same PR)

My next PR will rewrite the FileStream to use the Morsel API

## Are these changes tested?

Yes by existing CI.

I will work on adding additional tests for just Parquet opener in a
follow on PR

## Are there any user-facing changes?
No

Author: alamb