Refactor websubhub subscription/unsubcription flow

### Description

> $subject

This logic refactoring is required to implement the [Subscription autoverification](https://github.com/ballerina-platform/ballerina-spec/blob/c2742e84f1a36cb5afea3974af1e4f86b07f86ca/stdlib/proposals/websubhub/subscription_autoverification.md) feature.

### Version

_No response_