Auto merge of #16840 - Wilfred:shell_runnable, r=Veykril

Allow rust-project.json to include arbitrary shell commands for runnables

This is a follow-up on #16135, resolving the feedback raised :)

Allow rust-project.json to include shell runnables, of the form:

```
{
  "build_info": {
    "label": "//project/foo:my-crate",
    "target_kind": "bin",
    "shell_runnables": [
      {
        "kind": "run",
        "program": "buck2",
        "args": ["run", "//project/foo:my-crate"]
      },
      {
        "kind": "test_one",
        "program": "test_runner",
        "args": ["--name=$$TEST_NAME$$"]
      }
    ]
  }
}

```

If these runnable configs are present for the current crate in rust-project.json, offer them as runnables in VS Code.

This PR required some boring changes to APIs that previously only handled cargo situations. I've split out these changes as commits labelled 'refactor', so it's easy to see the interesting changes.

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