Add Span#deactivate to the public API

Currently, the public API does not allow for `deactivation` of active Transactions and Spans arbitrarily. The only way to deactivate is by keeping the `Scope` reference returned from the `activate` API.

An example where this would be required - [this discuss topic](https://discuss.elastic.co/t/spring-kafka-instrumentation-kafkalistenerendpointcontainer-mono-thread/193810).

I also found this limiting when implemented [this context propagation example](https://github.com/elastic/apm-agent-java/blob/24f60f9fcef59c58ad99d848efc28f14d6c772d1/apm-agent-plugins/apm-api-plugin/src/test/java/co/elastic/apm/api/BlockingQueueContextPropagationTest.java)