Add ability to add custom stacktraces

Hi,

# Summary
Add ability to add custom stacktraces to the ones that sentry-go is finding automatically

## Motivation
I'm using github.com/ztrue/tracerr that contains all my traces when anything goes wrong.

I'd like to add that information to Sentry, as a stacktrace. The information is not as accurate as the one from tracerr.

I'm currently pushing it as breadcrumbs, which is definitely not ideal (especially since we can't "clear" breadcrumbs)

##

Thanks!

PS : it might be out of scope, I might misunderstand something about the plugin. If so, I'm sorry :)