Reduce weighing overhead for caching blocks

[scwhittle]

Based upon profile we are currently deeply weighing ArrayLists containing elements.
This means that:

• we don't use the Weighted optimization for elements contained by the list
• we have additional weighing overhead of the ArrayList implementation

Instead modify blocks to use immutablelist which allows us to use array size calculation and just weigh individual elements.

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[gemini-code-assist]

Summary of Changes

Hello @scwhittle, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a significant optimization to the caching mechanism within Apache Beam by refining how the weight of cached blocks is calculated. By transitioning from mutable ArrayLists to ImmutableLists and adopting a more precise, less overhead-intensive weighing strategy, the changes aim to improve performance and stability, especially when dealing with large collections of elements in the cache. The update also incorporates a robust method for handling weight overflows.

Highlights

• Optimized Weight Calculation: Switched from deep weighing ArrayLists to calculating weight based on ImmutableList size and individual element weights, significantly reducing overhead.
• ImmutableList Adoption: Replaced ArrayList with ImmutableList for block storage, enabling more efficient weight calculation and potentially improving immutability guarantees.
• Saturated Addition for Weights: Implemented LongMath::saturatedAdd for weight accumulation to gracefully handle potential arithmetic overflows, preventing ArithmeticExceptions.

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