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 https://github.com/jazzband/django-model-utils/pull/556.

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