Move package depends to suggest

[sbfnk]

Description

This PR closes #796.

It makes the {future} packages and {progressr} suggests instead of depends, and only uses their functionality if available.

Probably the same could be done for {futile.logger} but that would be a bit more complicated as its functions are called a lot more than the ones moved here.

Initial submission checklist

• My PR is based on a package issue and I have explicitly linked it.
• I have tested my changes locally (using devtools::test() and devtools::check()).
• I have added or updated unit tests where necessary.
• I have updated the documentation if required and rebuilt docs if yes (using devtools::document()).
• I have followed the established coding standards (and checked using lintr::lint_package()).
• I have added a news item linked to this PR.

After the initial Pull Request

• I have reviewed Checks for this PR and addressed any issues as far as I am able.

[seabbs]

Nice. LGTM