why internal Provider？

[tianjingle]

When I customized a authentication , I went to implement the Provider interface and found that the internal was used in Pulsar-Client-Go, so I couldn't customize the authentication. How do I implement custom authentication for this piece? Looking forward to your answer. Thank you.

func NewAuthenticationIAM(ak string, sk string) auth.Provider {
	var provider auth.Provider
	provider = &IAMProvider{
		ak:        ak,
		sk:        sk
	}
	return provider
}

[nodece]

Could you make a PR to fix this?