Fix/deserialization of callback events

[Dovchik]

https://tickets.sinch.com/browse/DEVEXP-622
Add JsonPropertyName for hooks models where it was missing. This is required if System.Text.Json.Serializer is used.
Add methods to Conversation.Webhooks.Deserialize to help deserialize hooks out of the box.
Add tests for hooks deserialization

[asein-sinch]

The updates in this PR are going to the right direction, but in the current state, it's difficult to provide an approval:

• the referenced ticket is too vague about what needs to be done
• the PR is targeting the main branch which will trigger a new release upon merge. We have seen the difficulty to communicate about a version upgrade in the past, it would be better to cover all the deserialization cases in a single release
• to updated code is handling only a subset of messages types. Can you clarify the scope of this PR and confirm whether the other types of messages need to be updated or not?
  The E2E tests targeting the mocks from our official mockserver are passing because only a few types of messages are present. It would be better to add unit tests for this kind of code update.

[Dovchik]

Thanks for the review!

the referenced ticket is too vague about what needs to be done

Updated, it is more clear?

the PR is targeting the main branch which will trigger a new release upon merge. We have seen the difficulty to communicate about a version upgrade in the past, it would be better to cover all the deserialization cases in a single release
to updated code is handling only a subset of messages types. Can you clarify the scope of this PR and confirm whether the other types of messages need to be updated or not?
The E2E tests targeting the mocks from our official mockserver are passing because only a few types of messages are present. It would be better to add unit tests for this kind of code update.

Updated the description, it's more clear?
Regarding tests, do you expect to see specifically added Unit tests for models not present in E2E mocks?