Enabling of Cargo features

[sgwilym]

Is your feature request related to a problem? Please describe.

I do not know how to specify which Cargo features I would like rust-analyzer to use. I think this might not be possible without an update to the extension.

Describe the solution you'd like

Project settings for this extensions could include a stringArray configuration item for rust-analyzer's rust-analyzer.cargo.features option.

Describe alternatives you've considered

I have looked for other solutions like setting environmental variables, but none seem to be available for this purpose.