OneShotOptimizer inconsistent treatment of tunable_values

[bpkroth]

The OneShotOptimizer currently supports providing

1. include_tunable_values which allows for a list of files with tunable_values to try.
2. tunable_values dictionary of a single set of tunable values to try.

However, the code only supports a single tunable_value assignment.

We should fix this to be consistent:

• either support multiple tunable values to be tried in a row (i.e. set max_iter == the len() tunable_values included or set in the config and rename the OneShotOptimizer to FewShotOptimizer or some such), or
• Only allow a single tunable value in the config as well.

_Originally posted by @bpkroth in https://github.com/microsoft/MLOS/pull/340#discussion_r1189033015_

[bpkroth]

@motus maybe related to #420 ?

[motus]

Yes, since merging #420 this is a non-issue as we initialize the tunables outside of the optimizer