Basic implementation of OCM 1.2 payloads

[glpatcern]

This PR includes:

• Adopting the new properties of the OCM 1.2 payloads, without implementing any new functionality for now. In particular, any non-empty requirement in a share will be rejected (a test was added for that).
• Extending the OCM discovery endpoint.
• Using the remote OCM discovery endpoint to establish the full URL of an incoming remote share, regardless if provided or not. When sending a share, though, we still send a full URL.
• Caching the webdav client used to connect to remote endpoints, with added compatibility to OCM 1.0 remote servers.
• Some refactoring and consolidation of duplicated code.
• Improved logging.

[update-docs]

Thanks for opening this pull request! The maintainers of this repository would appreciate it if you would create a changelog item based on your changes.

[glpatcern]

OK, this is good enough for now: we discover remote endpoints according to specs, but we still send full URIs to remotes. Also the integration tests assume full URIs, so the discovery part is not covered (but was tested independently).
In the future we may reconfigure the test to set the owner's address to localhost:<port> and pass relative URIs so to trigger discovery (but https is assumed when contacting the remote, so we'd have to configure TLS on the test as well).