Consolidate the options and backend_options configuration fields

[verveerpj]

Is your feature request related to a problem? Please describe.
The optimization section of the Everest config has both options and backend_options fields. The options field is used to pass options in the form of plain strings, while backend_options expects a dictionary of key/value options. Which one to use depends on the optimizer, i.e. Dakota uses options, while SciPy uses backend_options. The two options should be consolidated into a single option that accepts both forms.

Describe the solution you'd like
The Everest configuration is implemented using pydantic, which can easily be used to implement a field that accepts both strings and key/value dicts. The options field should be adapted to allow this. The backend_options should be retained for now, but deprecated. It may be convenient to just make backend_options an alias for the new options until it is removed.

Describe alternatives you've considered
N/A

Additional context
The ropt configuration already uses this consolidated form, hence the implementation should be straightforward.