Refactor to Facilitate Decoupling from Concrete Implementations of EventFormat

[skepticoitusInteruptus]

Closes #537

Example Usage of Proposed Refactor

import io.cloudevents.CloudEvent;
import io.cloudevents.core.format.EventFormatProvider;
import io.cloudevents.core.builder.CloudEventBuilder;
import io.cloudevents.core.format.ContentType; // New constant to facilitate loose coupling in client applications

CloudEvent event = ...;

byte[] serialized = EventFormatProvider
    .getInstance()
    .resolveFormat(ContentType.PROTO) // EventFormatProvider.resolveFormat(...) overloaded 
    .serialize(event);                // for string and enum input options

[JemDay]

It looks like you need to sign your 'git commit' to get the build to run.

I do like how this cleans up the dependencies, may need to tweak the documentation on the ContentType to make it clear it's only enumerating the types defined by the various CloudEvent formats.

[JemDay]

And it looks like you 'pushed' while I was commenting :-)

[skepticoitusInteruptus]

And it looks like you 'pushed' while I was commenting :-)

Yeah. I didn't intend to push so many commits. I tried (in vain) to rebase all but one of them away. But I ended up with even more than I started out with.

I suck at git. Haha!

[pierDipi]

Thanks!

LGTM