Support multiple api roots

[mdvorak]

1. Extract from ApiUrl abstract base class for DI
2. Optionally rename it to ApiMapper
3. Current functionality move to SingleApiRoot
4. Add optional NamedApiRoot with support for several endpoints

Note these will be exclusive - either you will use single root with no prefix, or named prefixes, which means there always have to be at least the prefix in the url.
For NamedApiRoot, one api should be still marked as root path handler, that is / will be translated to its prefix.

[mdvorak]

There is an issue with APP_API_PREFIX, which is declared globally, and won't be much use for NamedApiRoot.