Resolves#11318.
* Adds `TestConfig.attachments` public API. (We opted to not implement an analog to the async `TestInfo.attach(…)` API.)
* Adds `TestConfig.attachments` to common reporters.
* Dogfoods some git and CI-info inference to generate useful atttachments
* Updates HTML Reporter to include a side bar to present a pre-defined set of attachments (a.k.a git/commit context sidebar)
Here's what it looks like:
<img width="1738" alt="Screen Shot 2022-03-21 at 3 23 28 PM" src="https://user-images.githubusercontent.com/11915034/159373291-8b937d30-fba3-472a-853a-766018f6b3e2.png">
See `tests/playwright-test/reporter-html.spec.ts` for an example of usage (for dogfood-ing only). In the future, if this becomes user-facing, there the Global Setup bit would likely become unnecessary (as would interaction with attachments array); there would likely just be a nice top-level config and/or CLI flag to enable collecting of info.
