feat(replay-assertions): Add replay assertions POC

[suejung-sentry]

WIP

[getsantry]

This issue has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you remove the label Waiting for: Community, I will leave it alone ... forever!

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"A weed is but an unloved flower." ― Ella Wheeler Wilcox 🥀

[suejung-sentry]

keep branch alive please (will come back to this in a couple weeks after another project)