Add BufferExec execution plan

[gabotechs]

Which issue does this PR close?

• Closes #.

Rationale for this change

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

• Do not require mut in memory reservation methods #19759
• This PR
• Hash join buffering on probe side #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?

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?

yes, by new unit tests

Are there any user-facing changes?

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