Alter OncePerSparkSession to be instantiatable even without provided SparkSession

[benedeki]

Background

Currently OnlyOncePerSparkSession can be instantiated only when provided with SparkSession. This turns out to be limiting for classes that needs to be created before Spark is available.

Feature

• add a new constructor which takes no parameters
• keep the old constructor, including autoegistering step
• make register method public, while keeping the feature that actual registration happens only once per SparkSession

Up to consideration if to make the SparkSession parameter of the old constructor explicit, to avoid confusion when the (auto-) registration happened and when not.
On the other hand that would create a breaking change for all current users.

[benedeki]

Bonus
Please change the signature of the method register to return True if the actual registration happened or False if it has happened in an earlier call.
(I think it's such a small change, no point creating a dedicated ticket to it)