[User-Story] Detect pinning support

[PaulLeCam]

Rationale

Clients should have a simple way of checking if a Swarm server supports pinning content.
One way this could be achieved now is to do a GET bzz-pin in order to list the pins and detect if the response code is 403, but this seems wasteful for the Swarm server to return the list of pins when the client doesn't actually need it.

User-Story

As a dapp developer I would like to be able to detect if the Swarm server I'm connected to supports pinning content so I can adapt the UX accordingly.

Acceptance criteria

A HTTP endpoint over which to perform a HEAD or GET request to check if pinning is enabled.
It could possibly be covered under the HTTP endpoint for adaptive node capabilities as a single endpoint for all capabilities could be more useful to a client than making multiple requests for individual capabilities.

Related Issues

• Pinned content
• HTTP endpoint for adaptive node capabilities