feat(concepts): add Streams concept

[navaneedan07]

📘 Description

This issue proposes the addition of a new Streams concept to the Java track on Exercism.

🔹 Concept Overview

Java Streams offer a functional and declarative way to process collections. This concept introduces learners to:

• Stream creation from collections
• Common operations like filter, map, reduce, and collect
• Lazy evaluation and pipeline structure
• Real-world use cases for transforming and aggregating data

📁 Files Added

• concepts/streams/about.md
• concepts/streams/introduction.md
• concepts/streams/config.json
• concepts/streams/links.json

This concept is designed to be beginner-friendly while introducing powerful tools for writing expressive Java code.