Remove `Attributes` interface

[bsideup]

In the current version of SDK, one has to deal with generic signature of CloudEvent and an unnecessary allocation of the AttributesImpl object.

While working on CloudEvents for Liiklus, I had to implement our own CloudEvent type that implements both CloudEvent and Attributes to reduce the allocations (see the link)

Since (according to the spec) there is always an 1:1 mapping of an event and its attributes, I suggest to remove the interface and make the fields part of the CloudEvent interface.

To make the change less impactful and support a migration path, I suggest we do it in two steps (that should be released separately):

1. Make CloudEvent implementations implement both CloudEvent and Attributes and return this from getAttributes()
2. Make CloudEvent extend from Attributes and turn getAttributes() into a default method that returns this and deprecate getAttributes() and Attributes
3. Remove Attributes and ``getAttributes()and move the fields to theCloudEvent` interface

If the community agrees, I can work on all 3 PRs.

[slinkydeveloper]

Now that the generics problem doesn't exist anymore, I think this discussion really brings us down to the "inheritance vs composition" debate, which doesn't have a real winner here. I think we should keep as is for several reasons:

• Easier to implement conversions back/forth different spec versions
• Easier to implement Message interfaces
• Less burden to have a new spec version
• Only one CloudEvent interface impl that already handles all the CloudEvent various methods
• A user can handle Attributes separately to the Event and eventually cast down the specific AttributesImpl to handle specversion specific informations, and we can still hide the details of CloudEventImpl
• Most important IMO: consistency with other SDKs

From a usability POV, I think CloudEvent interface can extend Attributes and delegate the methods to AttributesImpl.