Add a new "callable" extension class

[M1ke]

This extension allows a custom callable, making it simpler for clients to do ad-hoc extensions in local scope without having to make specific extension classes for each use case.

For example a client could wrap their own container for the Sentry class, and set context on this as a user progresses through a flow. At the time an exception is handled they want current context passed in. They could set this up in advance using the callable extension like so:

$fn_release = static function (Event $event): void{
  $event->release = $event->release ?: self::getRelease();
};

return new CallableExtension($fn_release);

So the initial setup saves this extension, but if the context (in this case static context, for cross-cutting concerns) has new data by the time the stack is processed, the callable extension pulls that in.

(Originally at kodus/sentry#9)