feat: add constructor to set clientId on Client creation

[adenix]

Changes

This PR adds a new constructor to the Client class that allows setting the clientId at construction time.

Problem:
The Auth0 Management API allows users to specify a custom client_id when creating a new client. However, the current SDK does not provide a way to set this value without using reflection, forcing users to write code like this:

// Current workaround using reflection
Client client = new Client("My App");
// The Auth0 SDK does not provide a setter for 'clientId' though the
// API does allow setting a custom value.
Field clientIdField = Client.class.getDeclaredField("clientId");
clientIdField.setAccessible(true);
clientIdField.set(client, "custom-client-id");

Solution:
This PR provides a clean, type-safe way to set the clientId during construction without requiring reflection.

Classes and methods added:

• Added Client(String name, String clientId) constructor with JavaDoc

Summary of usage:

// Existing usage - still works (backward compatible)
Client client = new Client("My App");
// client.getName() == "My App"
// client.getClientId() == null

// New usage - can now set clientId at construction (no reflection needed!)
Client client = new Client("My App", "custom-client-id");
// client.getName() == "My App"
// client.getClientId() == "custom-client-id"

The clientId field remains immutable as intended - there is no setter method, and it can only be set during object construction using the new constructor. The existing single-parameter constructor remains unchanged and continues to work as before for complete backward compatibility.

References

• Management API Clients endpoint: https://auth0.com/docs/api/management/v2/clients/post-clients
• The API's client_id field is documented as: "The id of the client. If not provided, it will be auto-generated."

Testing

This change has been tested with comprehensive unit tests covering both the new functionality and backward compatibility:

• This change adds test coverage
  • Added shouldCreateClientWithNameOnly() to verify backward compatibility
  • Added shouldCreateClientWithNameAndClientId() to verify the new constructor sets both fields correctly
• This change has been tested on the latest version of the platform/language
  • All tests pass with Java 8+
  • Verified with ./gradlew test --tests ClientTest

Test coverage includes:

• Verification that existing single-parameter constructor still works as expected
• Verification that clientId remains null when using single-parameter constructor
• Verification that new two-parameter constructor sets both name and clientId correctly
• All existing tests continue to pass without modification

Checklist

• I have read the Auth0 general contribution guidelines
• I have read the Auth0 Code of Conduct
• All existing and new tests complete without errors

[adenix]

This adds support for #778

[tanya732]

LGTM !!

[tanya732]

Closing this PR, as fix has been released in v2.26.0 in #781