v0.3.0

• Full support for message blocks.
• SlackNet.AspNetCore allows registering handlers for block actions and providing options to external_select block elements.
• Added some registration method overloads to SlackNet.AspNetCore to allow more flexibility.
• Fixed DateTime properties, which were always null.
• RegisterDialogSubmissionHandler correctly registers the handler without requiring an extra manual registration.

Breaking Changes

• Attachment.Ts is now nullable to allow sending attachments with blocks.
• IMessageActionHandler.Handle no longer returns a MessageActionResponse, which didn't work anyway. MessageActionResponse has been removed.
• InteractionRequest.CallbackId property pushed down to old sub-types, since the new BlockActionRequest doesn't have it.
• DialogElement.Type is now a string, to allow extension if necessary. The DialogElementType enum has been removed.