Make outbox extensions dependency on tracing extension optional

Currently, the Debezium outbox extension for Quarkus has an unconditional dependency on the OpenTracing API: https://github.com/debezium/debezium/blob/master/debezium-quarkus-outbox/runtime/pom.xml#L26-L29. This shouldn't be the case, as the extension can also be used without OT, i.e this integration is an optional feature. The following needs to be done:

• Mark the dependency as optional in the pom.xml
• In the extension's source code, access and interact with OT classes only in case the OT API actually is present (because the user has added the Quarkus OpenTracing extension themselves); here is an example for a similar situation in the Quarkus Kafka Streams extension, which registers some beans only case MicroProfile Health is enabled