Consider using a `version` parameter on the IANA media type `application/activity+json`

[trwnh]

Currently, the use of the IANA media type application/activity+json comes with the expectation or implicit assumption that the content or document is using the semantics defined in the normative context available via https://www.w3.org/ns/activitystreams (by way of requirements in AS2-Core saying that AS2 processors using JSON-LD MUST assume the context if it is not present).

However, the normative context can change, as described in #659 -- and this can in turn change the semantics of documents that were authored assuming the old context. The best practice here is to only refer to frozen, immutable contexts (such as https://w3id.org/security/v1 vs https://w3id.org/security/v2).

Taking these two facts together, it makes sense to extend the IANA registration for the application/activity+json media type to include version as an "optional parameter": https://www.iana.org/assignments/media-types/application/activity+json