Save creation time of message in ChatMessage

[Szpoti]

Partially fixes #12429

This is only business code. The time of creation now gets saved in the ChatMessage object, for both user and agent messages, but it's not visible anywhere, that will be a UI task. @zkSNACKs/ui-team

What happened:

• Moved ParseRawMessage from AssistantViewModel and moved it to the base, MessageViewModel.
• This saves us from code duplication and will solve removing the ugly timestamp from every message.
• From some places I removed the setting of UiMessage, as now it's handled in the base class.
• Only exception is the OfferMessageViewModel, where it has to be overwritten.
• Added a small logic in WorkflowStep.EditMessageAsync so the timestamp from the raw text doesn't disappear.
• Expanded the ChatMessage record, so it reads DateTime from the Unix timestamp of the message.

[adamPetho]

Suggested change

	var timestamp = GetTimestampFromMessage(chatMessage);
	newMessage = newMessage with { Text = timestamp + userMessage };
	newMessage = newMessage with { Text = GetTimestampFromMessage(chatMessage) + userMessage };

Since we don't do any checks on timestamp, maybe we could simplify this. WDYT?

[Szpoti]

You're right, but I like my version slightly better for readability.

[soosr]

Sorry for the late comment, taking a look at the problem now I think it should be the following (Although this is just my opinion, a final decision is @molnard's):

1. Whenever we receive the raw string from SIB, during parsing it should also parse the time stamps and store it in ChatMessage's property, and store the message text without the @xxxxxx@ part.
2. The UI should not deal with parsing (For the release it was fine, but now we could do it properly) that is the business side's responsibility.
3. The UI, when adds a new message, should just set the timestamp property in ChatMessage.
4. When we send back the whole conversation, the business side should form it properly back to a raw string (get the timestamp from the property and paste in front of the message text in the correct format).