Add standalone Java API usage examples

[gavv]

What do we need:

• We should port sender and receiver examples that we already have for C API to Java.
• Examples should be buildable and runnable project(s).
• Examples should be minimal and simple. All steps should be documented, similar to our C examples.
• Examples should be interoperable with roc command-line tools (e.g. it should be possible to run example sender in Java with roc-recv tool as a receiver).

References:

• See "Quick start" section in README of this repo
• Examples for C API:
  https://roc-streaming.org/toolkit/docs/api/examples.html
• Tests for Java bindings: https://github.com/roc-streaming/roc-java/tree/main/src/test/java/org/rocstreaming/roctoolkit
• Android app written in Kotlin and using our Java bindings: https://github.com/roc-streaming/roc-droid