Integrate htgettoken invocation into osdf-client

[bbockelm]

The new OAuth2 integration (#27) to automate token acquisition is quite useful - but only useful if OAuth2 is used by the experiment!

A counterexample is IGWN (and JLab and Fermilab); they want to use htgettoken for their flows (for example, they don't want to enable dynamic clients on their issuer). We should be able to annotate the namespace as:

1. Using htgettoken instead of oauth2.
2. The desired value of HTGETTOKENOPTS for that part of the namespace.