Introduce a Merge Queue to Validate Combined Changes Before Landing

[suryaprasanna]

Feature Description

What the feature achieves:
Introduce a merge queue so each PR is tested against the actual state of master it will land on, including any changes queued ahead of it.

For example:

M + A    -> CI passes -> land A -> M = M + A, let's denote this by M'
M' + B   -> CI passes -> land B -> M = M' + B

This ensures that the exact combination of changes being introduced into master has passed the required checks.
Why this feature is needed:
Two PRs can independently pass CI and have no Git conflicts, but still break master when combined.

For example:

M       -> stable
M + A   -> CI passes
M + B   -> CI passes

does not guarantee:

M + A + B -> CI passes

A merge queue would catch these semantic or integration conflicts before the changes are landed.

User Experience

How users will use this feature:
No changes to User Experience, if CI breaks, users are supposed to rebase their changes and fix the CI and again go through the PR review process again.

Hudi RFC Requirements

RFC PR link: (if applicable)

Why RFC is/isn't needed:

• Does this change public interfaces/APIs? No
• Does this change storage format? No
• Justification: