Feature: Add $self for identity-based referencing

[handrews]

OAS 3.2 added the $self feature which sets the base URI and is used as the preferred $ref target for external referencing, which would include references from Arazzo. To have unity across the ecosystem and ensure that an Arazzo document can be packaged for location-independent use with OADs, Arazzo should add support for a 3.2-like $self field and adopt similar URI / identity-based-referencing language as OAS 3.1 and 3.2.