Enable an offline mode that passes --offline to cargo

[tgross35]

When working in slow or no internet situations, RA seems to lock the registry/workspace trying to do internet things. It can be tricky to cancel these operations.

My suggestion is to add a new offline mode that can be toggled via the menu. This can either set the environment variable (CARGO_NET_OFFLINE=true) or pass --offline to each cargo command (I'd prefer this so it's easy to tell the mode is being used). This would be two new menu options:

• "rust-analyzer: Enable offline mode": cancel any running operations, set the flag, and relaunch anything in progress
• "rust-analyzer: Disable offline mode": allow running operations to complete, remove --offline flag

[tgross35]

Related, #12499

[clouds56]

As related, I tried to add --offline in settings,

but it seems not have any effect on cargo metadata

$ ps -ef |grep cargo
  501 82674 79393   0 10:58AM ??         0:00.97 ~/.rustup/toolchains/stable-aarch64-apple-darwin/bin/cargo metadata --format-version 1 --manifest-path /path/to/Cargo.toml --filter-platform aarch64-apple-darwin
  501 82909 74547   0 10:58AM ttys027    0:00.00 grep --color=auto cargo

[Veykril]

cargo metadata is not configurable by r-a currently. Also see #12499