Only update benchmark weights when there is a significant change

[teor2345]

There is noise in benchmark runs that results in a lot of trivial weight changes every time we re-run a benchmark.

To reduce benchmark churn, and focus reviewer time on significant changes, any trivial changes should not be added to the PR.

This could be implemented in the benchmark code generator, or as a post-run script that removes trivial changes from the git diff.

[teor2345]

Upstream issue and discussions:

• Add support for hysteresis thresholds for benchmarks paritytech/polkadot-sdk#826
• update weights paritytech/polkadot#5507 (comment)

This site does weight comparisons for upstream:
https://weights.tasty.limo/

Example report:
https://weights.tasty.limo/compare?repo=polkadot-sdk&threshold=5&path_pattern=.%2F**%2Fweights%2F**%2F*.rs%2C.%2F**%2Fweights.rs&method=asymptotic&ignore_errors=true&unit=time&old=polkadot-v1.16.4&new=update-weights-weekly-2025-02-24-1740411169

Here's how they create weights PRs in CI:
https://github.com/paritytech/polkadot-sdk/blob/1445af004bcac0ec3dd7b055a04bb1d7f9c110f1/.github/workflows/bench-all-runtimes.yml#L142

And here's an example weights PR:
paritytech/polkadot-sdk#7704