Fix HostAuthenticationStateProvider to use relative return URLs

[kevinchalet]

Fixes #382.

[JJong-nl]

ps : one small naming.
For prerender/server render blazor is using the normal ServerAuthenticationStateProvider as it should.
And rendermodes can be set to Server of webassembly.
Instead the HostAuthenticationStateProvider maybe namechange to WasmAuthenticationStateProvider?

[JJong-nl]

Than for the httpclients that using yarp forwarding. In wasm the cookie is coppied by the baseaddress. But when using server prerender a httpclient that should call the yarp forwarding should need the current cookie added to the server request. Can be done by request options and httpmessage middleware.

If you would like, i could sens sample