Add quarkus-resteasy-reactive dependency to build gradle

[dosera]

Motivation

While the build works with maven (and the dependency was added with aede7e08) it currently fails with gradle:

./gradlew clean build
..
src/main/java/org/jboss/aerogear/keycloak/metrics/MetricsFilterProvider.java:18: error: cannot find symbol
..

What

Adds the quarkus-resteasy-reactive to the build.gradle.

Why

Build was unable to complete successfully.

How

Adding to build.gradle.

Verification Steps

1. ./gradlew clean build

Checklist:

• [x] Code has been tested locally by PR requester
• [ ] Changes have been successfully verified by another team member

Progress

• [x] Finished task