Add BufferExec execution plan (#19760)

## Which issue does this PR close?

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- Closes #.

## Rationale for this change

This is a PR from a batch of PRs that attempt to improve performance in
hash joins:
- #19759
- This PR
- #19761

It adds a building block that allows eagerly collecting data on the
probe side of a hash join before the build side is finished.

Even if the intended use case is for hash joins, the new execution node
is generic and is designed to work anywhere in the plan.

## What changes are included in this PR?

> [!NOTE]
> The new BufferExec node introduced in this PR is still not wired up
automatically

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Adds a new `BufferExec` node that can buffer up to a certain size in
bytes for each partition eagerly performing work that otherwise would be
delayed.

Schematically, it looks like this:

```
             ┌───────────────────────────┐
             │        BufferExec         │
             │                           │
             │┌────── Partition 0 ──────┐│
             ││            ┌────┐ ┌────┐││       ┌────┐
 ──background poll────────▶│    │ │    ├┼┼───────▶    │
             ││            └────┘ └────┘││       └────┘
             │└─────────────────────────┘│
             │┌────── Partition 1 ──────┐│
             ││     ┌────┐ ┌────┐ ┌────┐││       ┌────┐
 ──background poll─▶│    │ │    │ │    ├┼┼───────▶    │
             ││     └────┘ └────┘ └────┘││       └────┘
             │└─────────────────────────┘│
             │                           │
             │           ...             │
             │                           │
             │┌────── Partition N ──────┐│
             ││                   ┌────┐││       ┌────┐
 ──background poll───────────────▶│    ├┼┼───────▶    │
             ││                   └────┘││       └────┘
             │└─────────────────────────┘│
             └───────────────────────────┘
```

## Are these changes tested?

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yes, by new unit tests

## Are there any user-facing changes?

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users can import a new `BufferExec` execution plan in their codebase,
but no internal usage is shipped yet in this PR.

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Author: gabotechs