Provide a means of setting the toolchain used by rust-analyzer in VSCode

[Veykril]

I've been reminded how with the python vscode extension you can select the interpreter used from a drop down list, it feels like we should be able to do something similar for the rust toolchain used

Ideally this would only live in the extension, but the toolchain choice is a per workspace thing and so likely needs integration with either rust-analyzer.toml(which does not yet work that well) or interfacing directly with toolchain overrides. Though the rust-analyzer used is not a per toolchain choice, so it might suffice to set it "globally", force the server from the toolchain to be used etc.