Add #to_h to models; make #inspect dump all attributes

[shalvah-gs]

Port of koshilife#64

This PR aims to improve the debugging experience and general UX by making it easier to see what's going on.

First, adding a #to_h method on Calendly models, so you can easily see the raw data as a hash (helpful for generating API payloads, for instance).

Second, increasing the usefulness of the #inspect method by making it print all attributes. Previously, this was what you'd get:

> pp event
=> #<Calendly::Event:27980 uuid="c79022c3-45a1-46e9-96bd-8fab44908b48", name="Testing/Staging ", status="active", ..>

This was super frustrating, as the truncation was hardcoded, and there was no way to see all the attributes at once.

Now you get:

> pp event
=> #<Calendly::Event:27980 uuid="c79022c3-45a1-46e9-96bd-8fab44908b48", name="Testing/Staging ", status="active", created_at=2023-06-29 09:23:34.003966 UTC, end_time=2023-06-30 08:00:00 UTC, event_guests=[], event_memberships=[#<Calendly::User:27900 uuid="0c5d2a34-4113-4cff-9b34-7db34c8dc324", name=nil, slug=nil, email=nil, uri="https://api.calendly.com/users/0c5d2a34-4113-4cff-9b34-7db34c8dc324">], event_type=#<Calendly::EventType:27920 uuid="bc2baf5b-9aae-42b1-8438-6d3079da163c", name=nil, type=nil, slug=nil, active=nil, kind=nil, scheduling_url=nil, uri="https://api.calendly.com/event_types/bc2baf5b-9aae-42b1-8438-6d3079da163c", ..>, invitees_counter=#<Calendly::InviteesCounter:27940 active=1, limit=1, total=1>, location=#<Calendly::Location:27960 type="custom", status=nil, location=nil>, start_time=2023-06-30 07:30:00 UTC, updated_at=2023-06-29 09:23:34.923187 UTC, uri="https://api.calendly.com/scheduled_events/c79022c3-45a1-46e9-96bd-8fab44908b48">

Much longer, but much more helpful, as intended for #inspect.