Refactor `HttpServiceClientAutoConfiguration` and
`ReactiveHttpServiceClientAutoConfiguration` to support scanning for
`@HttpServiceClient` annotated interfaces.
Closes gh-46782
The enum itself is not specifically tied to auto-configuration so
spring-boot is a better place for it. The related condition,
`@ConditionalOnThreading`, remains in spring-boot-autoconfigure. This
aligns things with the similar `CloudPlatform` and
`@ConditionalOnCloudPlatform`.
Closes gh-46406
Phillip Webb
Moritz Halbritter
Stéphane Nicoll
Andy Wilkinson