[Azure Container Registry] Expose authenticationClient/authenticationPolicy in containerregistry package

[MartinForReal]

Service Package Request

azure cloud provider team needs to migrate credential provider to track2 client. Credential provider will exchange token for kubelet when kubelet try to fetch image from acr.
acr token exchange logic has been implemented in authenticationClient.

azure-sdk-for-go/sdk/containers/azcontainerregistry/authentication_client.go

Lines 37 to 55 in 0eb9152

	func (client *authenticationClient) ExchangeAADAccessTokenForACRRefreshToken(ctx context.Context, grantType postContentSchemaGrantType, service string, options *authenticationClientExchangeAADAccessTokenForACRRefreshTokenOptions) (authenticationClientExchangeAADAccessTokenForACRRefreshTokenResponse, error) {
	var err error
	ctx, endSpan := runtime.StartSpan(ctx, "AuthenticationClient.ExchangeAADAccessTokenForACRRefreshToken", client.internal.Tracer(), nil)
	defer func() { endSpan(err) }()
	req, err := client.exchangeAADAccessTokenForACRRefreshTokenCreateRequest(ctx, grantType, service, options)
	if err != nil {
	return authenticationClientExchangeAADAccessTokenForACRRefreshTokenResponse{}, err
	}
	httpResp, err := client.internal.Pipeline().Do(req)
	if err != nil {
	return authenticationClientExchangeAADAccessTokenForACRRefreshTokenResponse{}, err
	}
	if !runtime.HasStatusCode(httpResp, http.StatusOK) {
	err = runtime.NewResponseError(httpResp)
	return authenticationClientExchangeAADAccessTokenForACRRefreshTokenResponse{}, err
	}
	resp, err := client.exchangeAADAccessTokenForACRRefreshTokenHandleResponse(httpResp)
	return resp, err
	}

I'm wondering if the client can be exposed to the public so that it can be reused. Thanks!

[jhendrixMSFT]

This is being tracked in #20571

I believe there is a plan to expose, just uncertain when it will happen.

@tadelesh FYI