Allow for setting HiGHS options in config file

[alexdewar]

The highs crate lets you set options for the underlying solver via the Model::set_option method.

The full list of options for HiGHS is available here.

In general, we don't want users to have to set these manually, though we may set some of them ourselves in MUSE2 if they're appropriate. However, it may be useful for debugging or experimenting to be able to fiddle with some of the options, without having to hack the source code. As we have a generic settings file anyway, we could let the user set any additional HiGHS-related options in there like so:

[highs_options]
log_to_console = true
output_flag = true

As some of these options could affect model results, they should probably be set in model.toml.