Allow to configure serialization format via webhook

Problem/Motivation

Module hardcoding default serializer used for webhook https://git.drupalcode.org/project/webhooks/-/blob/8.x-1.x/webhooks.modu...
Custom webhooks can't override this

Steps to reproduce

Proposed resolution

- Add configuration setting to webhook setting form to select serialization format
- Keep backward compatibility (add post_update hook to configure default serializer for existing config)

Remaining tasks

implement, add tests, commit

User interface changes

additional setting with default value

API changes

TBD

Data model changes

TBD