feat: Add Network Authentication & refactor AuthMethod

[SMadani]

This PR not only adds support for the "backend" Vonage Network Auth API, but also refactors the com.vonage.client.auth package. Most notably, it separates the TokenAuthMethod class into ApiKeyQueryParamsAuthMethod and ApiKeyHeaderAuthMethod to more clearly distinguish their roles. Consequently, the auth method usage in DynamicEndpoint implementations has been modified to accommodate this.

Furthermore, proper testing of the existing auth methods is now in place, along with a major refactoring of SignatureAuthMethod and the RequestSigning helper methods.