Move equivalent credential out of this spec

[OR13]

@tplooker I would like to pull the "LinkedIdentifier" credential out of this spec, and into its own one, so it can be made more generic, and used outside of HTTP / well known endpoints.

Can you commit what you have somewhere so we can adopt support for it in this spec?

Prefer you open a CCG work item for it.

[tplooker]

Here is a link to what I have documented to date https://hackmd.io/IZgDPFy6QiaZXbUk7Ik87Q

And an active gist under my GH showing the bi-directional linkage of a DID to a github account https://gist.github.com/tplooker/ca8c147906764afb26d95170900c560c

I believe @dmitrizagidulin had some commentary that he would like to use alsoKnownAs instead of sameAs?

[OR13]

I propose we create a seperate spec for the generic "indentifier link" credential in the ccg, and then use that credential here.