servant-client: replace BaseUrl with uri-bytestring

Looking at https://hackage.haskell.org/package/servant-client-core-0.13.0.1/docs/Servant-Client-Core-Internal-BaseUrl.html I can think of following benefits/reasons switching to https://hackage.haskell.org/package/uri-bytestring-0.3.1.1/docs/URI-ByteString.html#t:URIRef

a) haskell code around client can use `URIRef` type to construct the base url and pass it directly to servant-client

b) No changes necessary to servant-client, if for example one chooses to use basic auth (given that's it's mandated by proxy like nginx)

c) `uri-bytestring` is already a dependency of `http-api-data` that `servant-client-core` depends on

d) Removal of BaseUrl and the need to maintain it

Duplicates https://github.com/haskell-servant/servant/issues/139