Allow fetching of data by URL friendly name

Description

Support Friendly URL

• Currently, Headless delivers content through a url like http://<your-site-url>/api/episerver/v2.0/content/<contentReference>. However, a client may need to retrieve content through a "Friendly Url" as in CMS (for example, http://<your-site-url>/en/alloy-plan).

• Support CMS friendly urls, allow client to use these urls instead of ContentReference to retrieve content from Headless.
• Support to get children and ancestors by friendly URLs. For example, http://<your-site-url>/en/alloy-plan/children, http://<your-site-url>/en/alloy-plan/ancestors.

Requests should be routed

Request headers should have 'Accept' = 'application/json'; otherwise the request is ignored and not handled by content api route.

Content language

• Get language from request. Prioritize routed language over accept language header.
• If the request URL contains a specific language (for example: http://localhost:51059/en/alloy-plan/), the language, in this case 'en', should be used.
• If the request URL does not contain a specific language (for example: http://localhost:51059/), the language value specified in the Accept-Language header should be used.

Move RouteConstants to public namespace.
Still use endpoint version from v2.0 like previous versions.