Read and write extensions APIs

[slinkydeveloper]

Define APIs about how to read and write extensions

[slinkydeveloper]

As #108:

ExtensionsParser singleton is provided to read extensions:

DistributedTracingExtension ext = ExtensionsParser
  .getInstance()
  .parseExtension(DistributedTracingExtension.class, event);

While to write extensions, the CloudEventBuilder supports them:

builder.withExtension(ext);

[slinkydeveloper]

Extension needs to be reworked to remove the assertion that it can be translated to a map (aka similar to CloudEventExtensions, removing asMap).
Also readFromEvent should accept CloudEventExtensions, not CloudEvent