Make feed data consistent with booking endpoint data

[nickevansuk]

For oa:availableChannel the specification currently reads:

The channels through which a booking can be made must be included in Opportunity Data [RPDE] open feeds, but should not be included in other requests or responses.

This table should be updated to make this "optional" instead of "should not".

This allows an implementer to have a single translation layer between their internal data model and the OpenActive data model.

[nathansalter]

Completely agree with this. Essentially we can assume that most data providers will have a translation like this:

Where there's a single point where the objects in the Database are translated into the Openactive Models. If we need a further translation layer for each separate API, it increases complexity greatly so cases where this can be reduced would really help booking api creators.