Add new before_serialize_vars hook

[sl0thentr0py]

hmm @philipstarkey I see, that's a valid concern.

So the reason I looked at this again was because of a report where we patch the original f_locals and actually potentially break user code. We should generally not be holding the original variable references for longer than necessary.

Further, the scrubber was meant to be a simpler before_send with a simple denylist so that people can remove some vars by name easily from the event payload.

I'm going to go ahead with merging this PR, but at the same time I will expose another before_serialize_vars (naming TBD) that you can hook into to do precisely what you want in a separate PR.

Originally posted by @sl0thentr0py in #3409 (comment)

[sl0thentr0py]

closing in favor of #3427