Make OAuth2AuthorizationCodeGenerator public

[MaximeEsnol]

OAuth2AuthorizationCodeGenerator currently has package-private access. This makes it impossible to use it to manually generate an authorization code. There does not seem to be a particular reason for this class to be package-private, because other OAuth2TokenGenerator implementations, like JwtGenerator, OAuth2AccessTokenGenerator, and OAuth2RefreshTokenGenerator, are public already.

My proposed fix changes:

final class OAuth2AuthorizationCodeGenerator implements OAuth2TokenGenerator<OAuth2AuthorizationCode>

To:

public final class OAuth2AuthorizationCodeGenerator implements OAuth2TokenGenerator<OAuth2AuthorizationCode