feat: Add Author class and AuthorController

[mitchell-liatrio]

This pull request primarily introduces a new feature to the devopsknowledgeshareapi application: the ability to handle Author objects. This includes the creation of the Author class, an AuthorService to manage Author objects, and an AuthorController to handle HTTP requests related to Author objects. Additionally, a unit test for the AuthorController was added, and a small modification was made to the Post class to improve error messaging.

New feature - Handling of Author objects:

• src/main/java/com/liatrio/dojo/devopsknowledgeshareapi/Author.java: A new Author class was introduced, which includes firstName and lastName as attributes.
• src/main/java/com/liatrio/dojo/devopsknowledgeshareapi/AuthorService.java: An AuthorService class was added, which currently includes a stub for a method to get all authors.
• src/main/java/com/liatrio/dojo/devopsknowledgeshareapi/AuthorController.java: An AuthorController class was introduced, which includes a getAuthors method to handle HTTP GET requests at the /authors endpoint.

Testing:

• src/test/java/com/liatrio/dojo/devopsknowledgeshareapi/AuthorControllerTest.java: A unit test for the AuthorController was added, which tests the HTTP GET request at the /authors endpoint.

Improvement to existing code:

• src/main/java/com/liatrio/dojo/devopsknowledgeshareapi/Post.java: The error message in the setLink method was improved to include the invalid link.