This commit harmonizes the check for the presence of an Encoding bean.
Also, as this bean may be provided by another auto-configuration, this
commit orders them explicitly.
This means the dependency to spring-boot-zipkin has to elevate from a
test-only dependency to (at least) an optional dependency to validate
the links between the two auto-configurations.
Closes gh-46295
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
Stéphane Nicoll
Andy Wilkinson
Phillip Webb