Add error handler integrations option

[HypeMC]

Sometimes you might want to exclude the error handler integrations from the default ones in case you are using some other error handler implementation, eg a one provided by your framework (see getsentry/sentry-symfony#421).

One problem that having multiple error handlers may cause is having duplicate events in Sentry for the same event.

This PR introduces a new error_handler_integrations option which can be used to skip enabling the error handler integrations without the need to disable all default ones. When set to null, which is the default value, it'll fallback to using the default_integrations option. This is done to preserve backward compatibility.

[cleptric]

This should already be possible using Options::integrations, so I rather not add another option to achieve the same.

[HypeMC]

@cleptric Hi, while it is possible to do it the way you've suggested, it would require coping some of the code to mimic the default behavior and then also keeping that code in sync with any future changes. I was hoping to avoid that, but if you think it'd be better to solve it on the sentry bundle's side using this option, I'm ok with that.

[cleptric]

We're doing similar things in our Laravel SDK.
While I agree that this adds a bit of boilerplate to your application, I think this is also a use case not many people have.

There is definitely room for improvement on the Symfony SDK when it comes to the way we report errors, which is tracked via getsentry/sentry-symfony#337

[HypeMC]

@cleptric Thank you for the info, I'll close this one then.