Context escape with ScopeCallback

[marandaneto]

The callback typedef ScopeCallback = void Function(Scope);

Should be:

typedef ScopeCallback = FutureOr<void> Function(Scope);

Otherwise, there's a context scope because Scope has async methods but you can't await for configureScope.

void configureScope(ScopeCallback callback)

Should be:

FutureOr<void> configureScope(ScopeCallback callback)

Executions of the ScopeCallback? withScope should also be awaited otherwise the same problem happens.

Add changes to https://docs.sentry.io/platforms/dart/migration/#migrating-from-sentry-65x-to-sentry-66x