add support for LSP workspace/executeCommand message for initial use with Scala Metals

I'm using [Metals](https://scalameta.org/metals/) via https://github.com/dense-analysis/ale/blob/master/ale_linters/scala/metals.vim (thanks, @zoonfafer!)

Metals [needs users to import the build](https://scalameta.org/metals/docs/editors/vim.html#importing-a-build) upon first use within a workspace (i.e., a Scala project). Once you do that, the Metals server exposes all the goodies necessary for a rich user experience.

To tell the Metals server to import the build, you `POST` to its `/execute-command` endpoint with `command=build-import`:

```sh
curl -XPOST http://localhost:5031/execute-command?command=build-import
```

The first time one does this in a project, it takes a while. Subsequent invocations take much less time.

Is there some standard pattern for this sort of thing in ALE's plugin ecosystem?