Implement an ApplicationSource CloudEvents 1.0 http adapter

[devguyio]

Description

We need to create the adapter which will be deployed by the ApplicationSource controller. This adapter does the following :

1. It is configured with the correct Event Source in order to enrich the events.
2. It receives CloudEvents 1.0 events and sends it to its companion Channel.

Breakdown
Work left starting 11.11.2019

• Updating EventSource controller for integration with the adapter.
• Documentation

AC

1. There is an ApplicationSource adapter which gets created by the ApplicationSource controller.
2. The ApplicationSource adapter behaves as described above.
3. Documentations are updated.

[antoineco]

The following fields should be added to the HTTPSource spec as part of this implementation.

type HTTPSourceSpec struct {
    Source string
    // SourceID string (renamed)
    // optional
    MaxRequestSize int
}

[k15r]

SourceID should be renamed to Source as this is also the name of the field in the cloudevents specification.
Also it would be nice it we could limit the string to be a valid URI (same type as required by cloudevents spec)

[nachtmaar]

Open question: Do we expect that there is always a eventtypeversion set in the cloudevent? It might be required by the trigger to filter events based on eventtypeversion. eventype is required by the spec, eventtypeversion however is an optional extension.
The current implementation with event-service ensures that the eventtypeversion exists.

The combination of eventtypeversion and eventype is used in the UI to filter on events, e.g. v1.order_created vs v2.order_created.

If we want to rely on eventtypeversion for the lambda trigger, then we should validate it inside the adapter.

As discussed with @Abd4llA, will be done in a separate ticket: #6271

[nachtmaar]

As discussed with @Abd4llA and @bszwarc, developer document is delivered. Existing documentation (how eventing in kyma works) will be done in a separate ticket, since it is part of the whole broker/trigger epic.

---> #6312 documentation ticket.