feat(serializer): Allow classes to short circuit serializer with sentry_repr

[sl0thentr0py]

Based on the discussion in #1300 and #1296.

We do a lot of truncation magic in our serializer to not send huge payloads.
In the process, we ignore if a user/framework sets a custom __repr__ on a class.
This PR lets users add a sentry_repr that will short circuit our truncation logic and pick the user defined repr.
We cannot just go with checking if __repr__ is overridden because we still want to preserve the truncation behavior for classes out there in the wild like QueryDict.

Some notes from the TSC:

• for now this is python only because python is the only SDK with an elaborate serializer
• and in general, this only makes sense for dynamic languages

[antonpirker]

lgtm

[vmarkovtsev]

Thanks!

[sl0thentr0py]

@vmarkovtsev heads up, I had to change it to __sentry_repr__ in the latest release because we had some mocking problems on our end due to this change.

[vmarkovtsev]

Thank you for the update, very helpful!