Allow message unfurling with auth URL

[zerok]

This PR is basically a continuation of prior work by @kautsig in #839. I've just extracted his change into a separate branch for easier merging and added some test cases.

API changes

• Adds UnfurlMessageWithAuthURL method to the client
• Adds UnfurlMessageWithAuthURLContext method to the client
• Adds MsgOptionUnfurlAuthURL used within the two methods above

The Unfurl methods were added to be consistent with the UnfurlMessage method already exposed by the client.

[ds0nt]

Should also add these so I can authenticate users in unfurling flows as described here:

flow:

https://api.slack.com/reference/messaging/link-unfurling#authenticated_unfurls

things still to add:

https://api.slack.com/methods/chat.unfurl#arg_user_auth_required
https://api.slack.com/methods/chat.unfurl#arg_user_auth_message

[ds0nt]

"Specifying user_auth_url or user_auth_message will automatically imply user_auth_required being set to true. If both user_auth_url and user_auth_message are provided, user_auth_message takes precedence."

[zerok]

@ds0nt Good point. I'll add them on Monday to the PR 🙂

[zerok]

@ds0nt done :)

[kanata2]

LGTM! Thank you.

[ds0nt]

Thanks! :D!