fix: allow default_integrations to actually turn off the defaults

[rabbiveesh]

• in addition, we provide the defaults as a sub in case the user wants
  to manually add them (delaying initialization could make a difference)

If there's anything you'd want changed in the implementation, i'm happy to do so. I
personally need this b/c I used to get memory leaks from spans very early in my mojo app,
so i had to patch the code locally to turn off storing any breadcrumbs outside of the
req/res cycle.

In any case, the docs claim that you can turn off the default integrations, and before
this patch that is not the case.

I also changed the default value for default_integrations to 1 rather than empty array
b/c it's not meant to be an array