Improve root path lookup for OGC API endpoint

[jahow]

When using the OgcApiEndpoint ogc-client will look for the root path of the endpoint by going up one level in the URL until it finds a JSON document that looks like the root path.

This PR:

• clarifies the error given back by the library when encountering an issue during the root path lookup: either a 404 was received, invalid JSON was received, missing rel types etc.
• makes adaptations to have the library work for another implementation of the standard (geOrchestra Data Api)
• makes it so that ogc-client will add a trailing slash when querying the root path of an endpoint; this is because in some cases (e.g. a webapp deployed on a context) not including a trailing slash might result in a 404

Tests and fixtures were added for all this.

[cmoinier]

Thanks for these changes 🌠 Works well !