Client setup: remove old code and promote best practises

[gcatanese]

This PR improves the setup of the Client object required to perform the API requests. The following changes have been introduced:

• remove several legacy methods, deprecated already for 4-5 years

// removed as this method never worked (see README to follow best practises for Client setup)
public Client(String username, String password, Environment environment, String applicationName)

// deprecated since version 1.6.1 (see README to follow best practises for Client setup)
public Client(String username, String password, Environment environment, int connectionTimeoutMillis) 

// deprecated since version 1.6.1 (see README to follow best practises for Client setup)
public Client(String username, String password, Environment environment, int connectionTimeoutMillis, String liveEndpointUrlPrefix)

// deprecated since version 1.6.1 (see README to follow best practises for Client setup)
public Client(String apiKey, Environment environment, int connectionTimeoutMillis) 

// deprecated since version 1.6.1 (see README to follow best practises for Client setup)
public Client(String apiKey, Environment environment, int connectionTimeoutMillis, String liveEndpointUrlPrefix)

// deprecated since version 1.5.4 (see README to follow best practises for Client setup)
public void setEnvironment(Environment environment)

• refactor ClientTest to show case best practises for setting up the Client object

// setup using Config object
Config config = new Config()
    .environment(Environment.LIVE)
    .liveEndpointUrlPrefix("myCompany")
    .apiKey(apiKey);
Client client = new Client(config);

• add Javadoc to Client, 'ServiceandConfig` classes
• update README to show case best practises for setting up the Client object

[gemini-code-assist]

Summary of Changes

Hello @gcatanese, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request focuses on improving the Client object setup within the library. It streamlines the client initialization process by removing outdated methods and standardizing on a Config object for configuration. Additionally, it significantly enhances the codebase's documentation through Javadoc additions and updates the public-facing README to guide users towards these new best practices, ultimately leading to a more robust and user-friendly client integration.

Highlights

• Code Cleanup and Modernization: Removed several legacy and deprecated Client constructors and the setEnvironment method, simplifying the API surface and promoting current best practices.
• Configuration Standardization: Promoted the use of the Config object for Client setup, allowing for a more structured and flexible way to configure the client, as demonstrated in updated README.md examples and ClientTest.
• Improved Documentation: Added comprehensive Javadoc comments to the Client, Service, and Config classes, enhancing code clarity and maintainability for developers.
• Documentation Updates: Updated the README.md to reflect the new Client setup best practices, including examples for API key and basic authentication, and added a new section on error handling.
• Enhanced Test Coverage: Introduced a new ServiceTest.java file with dedicated unit tests for the Service.createBaseURL method, ensuring correct URL construction across different environments and API types.

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[gemini-code-assist]

Code Review

This pull request does a great job of cleaning up the client setup by removing deprecated methods, adding Javadoc, and updating documentation to reflect best practices. The changes significantly improve the library's usability. My review focuses on a few areas: ensuring code examples in the documentation are self-contained, maintaining consistent code formatting across the project, and adopting more modern testing patterns for improved readability.