Add ability to create custom markers/bookmarks in the header timeline

Requested by Mike Conley: "I'd love it if I could add my own markers to a profile while I'm analyzing it - like a bookmark, to know where things seem to be occurring. I usually need something like that if I'm trying to zoom on the region that I care about, and have only found one end of it so far."

More info: "Yep, timeline would be fine, certainly. So long as I could map it to the
samples in the threads up above, that's all I care about. :)"

* [ ] write about the user flow could work
* [ ] get feedback
* [ ] create initial wireframe

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-658)