Add Browser UserAction Event

[Karlie-777]

Fixes #

Changes

Please provide a brief description of the changes here.

Note: if the PR is touching an area that is not listed in the existing areas, or the area does not have sufficient domain experts coverage, the PR might be tagged as experts needed and move slowly until experts are identified.

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• schema-next.yaml updated with changes to existing conventions.

[breedx-splk]

I think this is good, especially as a first pass "development" event, thanks! I left some comments and think there is room for improvement and possibly sharing/broadening, but I don't want those to block. Thanks for making this bite-sized!

[trask]

Suggested change

	note: Add browser and enduser events.
	note: Add `browser.user_action` event.

[trask]

blocking temporarily to discuss whether we can unify screen coordinate representation with #2070

I've added this topic to next week's client instrumentation SIG meeting agenda