Add support for baggage in tracing

[mhalbritter]

Sleuth supports three types of baggage:

• local baggage, which is available only in the same JVM
• remote baggage, which gets propagated via network to other services (e.g. via HTTP headers)
• correlated baggage, which can be local or remote baggage, too. This baggage gets automatically synchronized to MDC

Trace id and span id are a "special" kind of baggage, which is needed for span propagation to other services.

As of now, we don't have support for that feature in Spring Boot, and should add it.

[wilkinsona]

Superseded by #32480.