Use self-reported operation URLs from capability documents

[ejn]

OWS Capability documents include definitions of the URLs to be used for all operations. As a general rule these are (i) all the same and (ii) the same as the URL from which the capabilities document was retrieved. This is however not necessarily the case, e.g. where request forwarding has occured. Formally, these self-reported URLs should be used for all operations.

This PR therefore exposes the self-reported operation URLs in the API and also uses these as the base for generated request URLs.

Notes:

1. some changes in some existing tests were necessary as "fake" URLs are used for mock requests, which are replaced by the self-reported URLs once the capabilities have been replaced.
2. Tests for the capabilities URL before the loading has completed (which will return a value based on using the request URL passed to the constructor and not the self-reported URL) are marked as skipped as due to the mocking the async calls actually complete synchronously and it is not currently possible to test the state before the capabilities were loaded.