Convert share_objectives feature into a runtime option

[BAGUVIX456]

Had a quick glance at the codebase and the documentation, and saw no cases where features are handled at runtime within the crate. I don't think we can use libafl_bolts' cli features in this case. Passing the option to share objectives to the required methods as an argument could be one way, but even that doesn't seem to be the most optimal way. RFC

[domenukk]

The user of the library will have to decide how/if they want to expose this somewhere. I mainly care about having this as option in code vs as a feature in Cargo.toml