Maybe add RxJava2 dependencies

[pway99]

RxJava2

Micronaut no longer ships any reactive implementation as a default in any of our modules or core libraries. Upgrading to Micronaut 3 requires a choice to be made about which reactive streams implementation to use and then to add the relevant dependency to allow for usage of it with the framework.

For those already using RxJava3 or Project Reactor, there should be no changes required to upgrade to Micronaut 3. For those using RxJava2 and wish to continue using RxJava2, a dependency must be added:

<dependency>
    <groupId>io.micronaut.rxjava2</groupId>
    <artifactId>micronaut-rxjava2</artifactId>
</dependency>

In addition, if any of the Rx HTTP client interfaces were used, a dependency must be added and the imports must be updated.

<dependency>
    <groupId>io.micronaut.rxjava2</groupId>
    <artifactId>micronaut-rxjava2-http-clent</artifactId>
</dependency>