Consider how to support HTTP service clients without using @ImportHttpServices

[philwebb]

Our initial cut of HTTP Service clients doesn't support automatic classpath scanning and as such @ImportHttpServices is always required. We should consider if we want to try and support automatic detection based on a class-level annotation directly on the interface client interface.

Three options considered so far:

1. Do nothing and always require @ImportHttpServices
2. Add a dedicated @HttpClient annotation with support for the group
3. Support a special pseudo URL on @HttpExchange, something like @HttpExchange("clientservicegroup://mygroup")

Option 3 is prototyped in https://github.com/philwebb/spring-boot/commits/interface-client-7, but will require changes to Spring Framework.