Retrieve conversation list from ConversationStore and agent trait

[barryvdh]

This allows to get the list of the conversation from the ConversationStore and use this in the RemembersConversations trait.

This makes it possible to get the list of conversation to resume in the UI, without calling the database directly.

Note: this is a BC break for any other store implementations, so would require a SemVer bump (if anyone implements their own store).

[barryvdh]

Not sure if we want to return it as array like messages, or collection (from the trait).

And maybe we should include the created_at / updated_at?

[taylorotwell]

I wonder if this should be traits you can add on to your User model for example that allows you to access the conversations via Eloquent? For both conversations and conversation messages.

[barryvdh]

I guess that would make sense and make it easier to handle the conversations/messages