Provide base route 2

Simpler take on https://github.com/7digital/SevenDigital.Api.Wrapper/pull/186

A toolkit to address https://github.com/7digital/SevenDigital.Api.Wrapper/issues/36 - "Enable using different API urls for each endpoint"

Removed it from the endpoint DTO, -  is not going to be useful as the DTO will usually come in a library & the caller will be wanting to modify the base uri after the fact using `Api.WithBaseUri(....`

This makes RouteParamsSubstitutor simpler as well - it just delegates the base uri to whichever BaseUriProvider is in use. The default BaseUriProvider does the logic of selecting secure or regular api uri