Allow other modules to extend StripeWebformHandler

Problem/Motivation

In its current state, module allows overriding the customer create object and subscription create object via YAML definitions in Handler config. But this doesn't allow conditional overriding.

And Example of this would be a webform with multiple subscription options, of which user can choose one.

We should allow users to override these objects.

Steps to reproduce

Proposed resolution

Add alter hooks.

Remaining tasks

User interface changes

API changes

Data model changes