feat: user-facing part of variable vnode count

[BugenZhao]

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What's changed and what's your intention?

This is a progress towards #15900.

Significant changes in this PR:

• The vnode count for singletons will always be 256 (no matter existing or newly created), instead of 1 used in previous PRs. This is to simplify backwards compatibility.
• Add session variable named streaming_max_parallelism to decide the max parallelism used in future streaming jobs. Note that the default value of the session variable can be changed globally with ALTER SYSTEM.
• Retrieve correct value of vnode count from expression context in rw_vnode(), used internally for 2-phase aggregation.
• Rename VirtualNode::COUNT to VirtualNode::COUNT_FOR_COMPAT, indicating that it should not be used in most of the code. Also update the Java binding function.
• Other minor fixes of TODOs.

The variable vnode count feature is now generally available after this PR, with just a few individual TODOs that still need to be fixed.

Checklist

• I have written necessary rustdoc comments
• I have added necessary unit tests and integration tests
• All checks passed in ./risedev check (or alias, ./risedev c)

Documentation

• My PR needs documentation updates. (Please use the Release note section below to summarize the impact on users)

Release note

Introduce a new session config of streaming_max_parallelism. Update the doc of streaming_parallelism. Please refer to the rustdoc for documentation.

[BugenZhao]

• feat(frontend): show job's max parallelism in system tables #18672
• feat: user-facing part of variable vnode count #18515 👈
• feat(streaming): support up to 16-bit vnode count in row id gen #18529
• feat: support per-fragment vnode count #18444
• feat(storage): variable vnode count support #18415
• refactor(frontend): extract filling fields in fragment graph #18466
• main

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[fuyufjh]

Generally LGTM!

[fuyufjh]

I'm wondering the initial reason that we used 256 as Singleton's vnode_count here... What if we change it to 1? i.e. Which part of code rely on Singleton's vnode count being 256?

[BugenZhao]

I don't remember exactly, but I previously encountered some problems and found that using 256 for the singleton simplified things.

However, it's true that using 1 is much clearer and more consistent. Given that this PR has passed the tests, I'll make another attempt in future PRs.