Add micrometer instrumentation for transactions to spring-tx

[edudar]

With the migration from Spring Cloud Sleuth to Micrometer Tracing, the direction was to move instrumentation support to respective projects. Looking through the code, it doesn't look like support for transactions has been migrated to spring-tx.

The original tx support in Sleuth: https://github.com/spring-cloud/spring-cloud-sleuth/tree/3.1.x/spring-cloud-sleuth-instrumentation/src/main/java/org/springframework/cloud/sleuth/instrument/tx that supports imperative and reactive transaction managers.

[bclozel]

This was considered and declined in #28156 (comment)