Consider interoperability in our #annotations fragment syntax

## Feature Request Form
### Problem you are trying to address with this feature

Right now our bouncer links resolve to an #annotations fragment that passes the necessary annotations and search facets to our client. This syntax is a likely point of intersection with the broader movement towards an ecosystem of interoperable clients and services, and the links that use these fragments are likely to be embedded in the web in larger numbers-- including by those copying and pasting our own bouncer output instead of the initial bouncer syntax.

### Your solution

We might consider an #annotation syntax that would work alongside other link syntaxes for deep linking, such as in these W3C notes:

Selectors and States:
https://www.w3.org/TR/2017/NOTE-selectors-states-20170223/

And Embedding Web Annotations in HTML
https://www.w3.org/TR/2017/NOTE-annotation-html-20170223/

This syntax could direct supporting clients to fetch annotations from indicated services, with filters for users, groups, tags, urls or plain text search terms.

Note previous discussion here:
https://github.com/hypothesis/product-backlog/issues/975#issuecomment-478292789

And a broader discussion around deep linking in browsers here:
https://github.com/bokand/ScrollToTextFragment/issues/4

Start by reading the linked W3C notes on Selectors and States and embedding Web Annotations in HTML, then review the discussion in product-backlog issue 975 and the browser deep-linking discussion in ScrollToTextFragment issue 4. Done would require a decided interoperable #annotation fragment design covering annotation services and the proposed filters; no implementation files or tests are identified here.

Written by the indexing model from the issue text.