Add a pom.xml for org.eclipse.paho.sample.mqttv3app sample to make it maven compliant

# Problem Summary

Currently the `org.eclipse.paho.sample.mqttv3app` sample is not maven compliant i.e. doesn't have a project level `pom.xml` file, because of which if we are to use a specific sample file, we need to clone that specific file, edit the package, compile it and then run the sample.

Adding a project level `pom.xml` file will help us to compile the entire sample, install its dependencies and package it as a jar which then we can directly run.

# Tasks

- [ ] Create a project level `pom.xml` file, including all the dependencies and plugins
- [ ] Create a new `Main.java` file which will act as an entry point for our `pom.xml`
- [ ] Since we are having 3 java samples inside our `org.eclipse.paho.sample.mqttv3app` package, so we need to add arguments in our `Main.java` file so when we run our project jar, using these args we can run any sample that we want inside the package.
- [ ] Create a `Readme.md` file elaborating the instructions on how to build and run the sample.

Start by inspecting the three Java samples in the org.eclipse.paho.sample.mqttv3app package and determine how pom.xml, Main.java, and Readme.md should connect. Done means the project-level Maven build includes its dependencies and plugins, the packaged jar can select each sample through arguments, and the README explains how to build and run it.

Written by the indexing model from the issue text.