Consolidate all auth types under a single client class
One client class (GraceDb) will now be usable for all authentication types (X509, basic, and None). Users can provide credentials to the constructor directly (path to cert and key or proxy file, or username and password). If those are not provided, it will look for X509 credentials in environment variables and then in an expected default location. Basic auth credentials will be looked up in the default location for a .netrc file. The client instantiation will no longer fail if credentials are not found (unless the user explicitly sets the 'fail_if_noauth' property to True).
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