Hashmaps are one of the most versatile data structures in Rust, providing a way to store key-value pairs. They are part of the standard library and offer a convenient way to associate values with unique keys. In this challenge, you will implement a simple key-value store using Rust's HashMap.

The HashMap type is part of the std::collections module and allows you to perform operations such as inserting, retrieving, and removing elements efficiently.

You are tasked with implementing two functions for a key-value store:

1. insert_or_update: Inserts a new key-value pair into the HashMap, or updates the value if the key already exists.
2. get_value: Retrieves a value by its key. If the key is not present, return None.

You will write both functions to work with a HashMap<String, String>.

• Accepts a mutable reference to a HashMap<String, String>, a key of type String, and a value of type String.
• If the key exists in the map, updates its value.
• If the key does not exist, inserts the new key-value pair.

• Accepts an immutable reference to a HashMap<String, String> and a key of type String.
• Returns an Option<String>:
  • Some(value) if the key is present.
  • None if the key is absent.