Including Security namespace terms in the Activity Streams 2.0 context document, take two

So, in order to test the new extensions policy, and practice including 
an extension schema into the AS2 context document, I proposed using the 
Security Vocabulary:

https://w3c.github.io/vc-data-integrity/vocab/security/vocabulary.html

It's used by almost every ActivityPub implementation because it's 
required for HTTP Signature authentication 
<https://swicg.github.io/activitypub-http-signature/>.

In our last meeting, when this topic came up for discussion, Dmitri made 
the very reasonable point that the Security Vocabulary includes dozens 
of terms, and has been something of a catchall for VCs and DIDs. 
Including the whole vocabulary would be unwieldy and unnecessary. We 
instead decided to focus on the ActivityPub Miscellaneous Terms 
<https://swicg.github.io/miscellany/> context instead.

However, I haven't given up on the idea of including terms from the 
Security Vocabulary, since its context document URL is present in almost 
every AP document. I'd like to propose that, instead of including the 
/entire/ vocabulary, we only include the terms widely used in 
ActivityPub for HTTP Signatures:

  * publicKey
  * publicKeyPem
  * owner
  * Key

I think this would cover the needs of most AP developers, would simplify 
millions of documents, and would not overload our context document with 
lots of unused security terms.

If other terms from the Security Vocabulary become needed or necessary 
in ActivityPub again in the future, implementers can include its content 
document in their @context properties, and we can eventually include 
those terms into the Activity Streams 2.0 context document under the 
same policy.

I'd like to propose this step at our next meeting.

Evan