Add support for experimental event types

[magnusbaeck]

Description

The protocol repo will add support for experimental event types, signified with a pre-1.0 version (see eiffel-community/eiffel#343). These will need special treatment since backwards incompatible changes may occur at any time, so we'll have to generate one type per pre-1.0 event version.

Motivation

The points of having experimental event types is to allow experimentation and iterative development. This ability is limited if you can't use this SDK when processing such events in Go code.

Exemplification

The upcoming EiffelArtifactDeployedEvent (see eiffel-community/eiffel#322) will start its life as experimental, and we obviously want to support its use in any way we can.

Benefits

Equally easy to work with experimental events than any other events.

Possible Drawbacks

None apart from more complexity in the code generation.