Uses the default CDP location (~/.cdp/credentials) and input profile
name, or the default if the input profile name is null. Loading of the
credential file is deferred until getCredentials() method is called.
profileName - profile name to use, "default" is used if null
Returns CdpCredentials which the caller can use to authenticate a CDP
request. Each implementation of CdpCredentialsProvider can chose its own
strategy for loading credentials. For example, an implementation might
load credentials from an existing key management system, or load new
credentials when credentials are rotated.