Enhance multiple auth support and auth client

[ckganesan]

In this Pull Request, I've implemented enhancements to address a specific limitation:

1. While the user password grant type previously supported only the default client (cf), it now accommodates the use of alternative clients, which was not possible before. Now we can achieve like below

config.New("https://api.cf.com", config.ClientCredentials('test', 'test'), config.UserPassword('test', 'test'))

2. At present, duplication of the client request body occurs when utilizing bytes.Buffer, bytes.Reader, and strings.Reader.

3. Reattempt the client request following the expiration of the refresh token.

4. Eliminate the use of loops in implementing retry logic.

[sneal]

I'll go ahead and merge this, any clean up (which is minor) can be done later