Support `BMI` exchange of wflow variables through listing of model variables in the TOML file

[verseve]

Feature type

Changing existing functionality

Improvement Description

The variables that wflow can exchange through BMI are now based on the different model components, these can be listed under the API section of the TOML configuration file. The variables that can be exchanged for each model component depend on the @exchange metadata setting for each field in a wflow struct. For coupling applications (e.g. with OpenDA) it would make sense to also have the option to control the exchange at the model variable level, by listing these variables in the TOML file.

Implementation Description

Instead of components have the option to use variables in the configuration TOML file, and change function BMI.get_input_var_names to support this.

Additional Context

No response