Proper url spec for ocm server url

[MahdiBaghbani]

I'm intentionally not updating the Discovery endpoint (and any related fields) in this PR. This needs a brief design discussion first. For reference:

* REQUIRED: endPoint (string) - The URI of the OCM API available at
this endpoint.  Example: `"https://my-cloud-storage.org/ocm"`

Once we reach consensus, I'll submit a follow up patch with the corresponding edits.

This PR is a result of discussion in #244

[mickenordin]

Great 👍

[glpatcern]

Looks good! Just a couple of changes

[KrausMatthias]

Maybe we should add something in Discovery on how to handle https vs http. Default to https only, rejecting any federated urls with http or OCM Servers only discoverable via http. Allow an Opt-In for http downgrades/fallback?

One might also leave that up to implementations, just something I've been thinking about.

[glpatcern]

Maybe we should add something in Discovery on how to handle https vs http. Default to https only, rejecting any federated urls

Yes, that's what Mahdi put in the subject of this PR, leaving Discovery as a separate point.

My take here would be to provide a recommendation (the obvious one...) and leave it to the implementations. De facto the only room for http is for testing purposes, and even there we have certificates and https in the OCM Test Suite, Paired with the Security considerations where we recommend httpsig, it does not leave much choice,