GUACAMOLE-2199: Adds support dynamic Base URL injection via configured header. Includes unit tests for API compatibility.

[rogerberaldi]

JIRA Issue: https://issues.apache.org/jira/browse/GUACAMOLE-2199

Overview:
This PR enables support for stateless load balancing architectures (e.g., Kubernetes Ingress, Zero-Trust Gateways) where sticky sessions are unavailable or unreliable. It introduces a configurable property that allows an upstream proxy to explicitly dictate the client's Base URL via an HTTP header.

Changes:

• Configuration: Introduced auth-base-url-header in guacamole.properties.
• API: Updated APIAuthenticationResult to include a baseUrl field.
• Compatibility: Implemented constructor chaining in APIAuthenticationResult to ensure binary compatibility with existing extensions.
• Logic: Updated TokenRESTService to inspect the configured header and inject the URL (replacing {TOKEN} placeholder) into the auth response.

Verification:

• Added APIAuthenticationResultTest to verify backward compatibility of constructors and correct injection of the baseUrl field.
• mvn test passed.
• Verified that default behavior (when property is unset) remains unchanged.