Support either UUIDs or URIs in client

[shalvah-gs]

In many places in the Calendly API and webhook payloads, a URI to a resource is returned. However, the client in this gem always expects a UUID, which forces users to either parse the URI and extract the relevant UUID parts, or use the model objects (an API which involves more boilerplate).

# Supposing, you have a Calendly invitee object; the old_invitee field is a URI

invitee_uri = invitee.old_invitee

# To fetch the old invitee, you can't do this:

Calendly::Client.new.event_invitee(invitee_uri) # <= doesn't work

# You can either write a regex to extract the UUIDs from the URI
event_uuid, invitee_uuid = /https:\/\/api\.calendly\.com\/scheduled_events\/(.+)\/invitees\/(.+)/.match(invitee_uri)
Calendly::Client.new.event_invitee(event_uuid, invitee_uuid)


# or (unnecessary boilerplate, must remember to `#fetch`)

Calendly::Invitee.new({ uri: invitee_uri }, Calendly::Client.new).fetch

This PR makes most of the client methods also accept URIs, while keeping backwards compatibility.