phpbench: added github action to perf compare PR changes against the main-branch

[staabm]

• a github action runs on new pull request
• the action will run the benchmark against the main branch and afterwards within the same instance check out the pull request contents and run the benchmark a second time -> a perf diff will be generated
• the resulting perf diff will be posted as a comment by the github action
• on followup commits the initial comment will be updated with a updated perf diff

[staabm]

blocked on upstream issue phpbench/phpbench#764

✔️

[staabm]

blocked by another upstream issue: peter-evans/create-or-update-comment#76

[staabm]

I guess this is ready to go. Whether this works like expected can only be tested after it was merged.

expected behaviour is

• a github action runs on new pull request
• the action will run the benchmark against the main branch and afterwards within the same instance check out the pull request contents and run the benchmark a second time -> a perf diff will be generated
• the resulting perf diff will be posted as a comment by the github action
• on followup commits the initial comment will be updated with a updated perf diff

#38 needs to be merged first as a preparation