Track historic best results and (optionally) compute ratio based on best results

[ksvladimir]

Thanks for providing this awesome action that our team greatly enjoys!

Our team got concerned that, since this action shows alerts based on comparison to the previous run only, it wouldn't catch gradual performance degradation over several commits. Such degradation is very real, especially when using this action for end-to-end benchmarks: the code becomes a bit slower (but below the alert threshold) with each new feature added, and eventually, it reaches the point where we want to revisit its performance — ideally as advised by an alert from this action.

This PR makes this action usable for detecting performance degradation over time in end-to-end benchmarks by:

• showing Best value for each benchmark in commit comments (in addition to previous and current values)
• having an option to use the best benchmark instead of the previous one for computing ratios and triggering alerts

We'd appreciate your consideration for including this PR in this action!

[ningziwen]

If it reaches a super good value by transient issue for one time, will be "Best" be corrupted and it will consistently alert until you fix the data point manually?

Alternatively, what do you think of setting an absolute threshold for your use case? Or thinking about a better stats value instead of "Best", like "Median during recent period"?