Add support for custom service contexts

[vlovgr]

Instead of always having a fixed ctx: Headers for generated service methods, we define Service.WithContext[F[_], A] with a custom context A. We let Service[F[_]] extends Service.WithContext[F, Headers] to keep source compatibility (but not binary compatibility). Existing Service.fromClient and toRoutes keep their shape, but we now also generate:

def toRoutes[F[_]: Temporal, A](
  serviceImpl: WithContext[F, A], 
  mkCtx: Request[F] => F[A]
): HttpRoutes[F]

def toRoutes[F[_]: Temporal, A](
  serviceImpl: WithContext[F, A], 
  mkCtx: Request[F] => F[A], 
  maxMessageSize: Int
): HttpRoutes[F]

and the following similar overloads for fromClient.

def fromClient[F[_]: Concurrent, A](
  client: Client[F], 
  baseUri: Uri, 
  mkHeaders: A => F[Headers]
): WithContext[F, A]

def fromClient[F[_]: Concurrent, A](
  client: Client[F], 
  baseUri: Uri, 
  mkHeaders: A => F[Headers], 
  maxMessageSize: Int
): WithContext[F, A]

Fixes #144.