Update generator throttler to support block-based strategy

[TheSafo]

What does this PR do?

Enables configuring byte-based throttlers in generators to be based per block (e.g. 10 blocks per second instead of 100 MB/s).

Impl. details:

• Adds a wrapper for generators around lading throttle that's based around throttling blocks at a time (which aligns with how the throttler is currently used)
• Adds some config tests to verify that both the new format and old format is still supported

Motivation

When using payload generators where blocks are intentionally not the same size/close to maximum bytes, it can be helpful to reason about throughput per block. This allows us to do things like:

• Define a payload where each block is 1 second worth of a system's logs
• Set the throttle rate at 5 blocks per second
  to simulate generating logs at 5x standard playback speed.

Related issues

A follow up pr (#1683) adds some flushing logic to BufWriter based generators that enables more regular flushing. A separate PR will introduce a new payload format to utilize block-based throttling.

[TheSafo]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• static-timestamped #1687
• Add flush_every_n_blocks config option to BufWriter generators #1683
• Update generator throttler to support block-based strategy #1675 👈 (View in Graphite)
• Add deserialization tests for BytesThrottleConfig and LoadProfile #1684
• Modify throttle errors to not assume capacity issue #1679
• Resize block buffer after allocation if significant wasted space #1674
• main

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