feat(integrations): add django signals_denylist to filter signals that are attached to by signals_span

[lieryan]

This PR adds a way to exclude sentry-sdk from attaching to certain Django signals. This can be used to work around performance issues when sentry attaches to performance sensitive signals like pre_init and post_init.

You pass a list of Django signals that you don't want Sentry to attach to:

import django.db.models.signals
import sentry_sdk

sentry_sdk.init(
    ...
    integrations=[
        DjangoIntegration(
            ...
            signals_denylist=[
                django.db.models.signals.pre_init, 
                django.db.models.signals.post_init,
            ],
        ),
    ],
)

This potentially fixes #1826 or #1739, and jazzband/django-model-utils#556.

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[wsantos]

Loved this, I have some init known signals that I'd like to disable mainly django-imagekit but not disable it all.

[sentrivana]

Hey @lieryan, thanks a lot for the PR and sorry for the wait! I'm looking at it now, will leave a review soon.

[sentrivana]

Looks good to me!

Not sure yet what's going on with the cache tests failing, fighting with that now, but as soon as I fix that I'll merge this. Thank you for contributing!