Xtask (#2688)

### Background
Comprehensive Rust requires a number of tools to be installed (e.g.
mdbook and mdbook-course). As mentioned in #2509 (and discussed in
#2469) it would be nice to have a cross platform command for installing
these dependencies. Currently these are installed using a shell script
(`install-mdbook.sh`) but this isn't truly cross platform e.g. for
Windows users.

### Xtask

[xtask](https://github.com/matklad/cargo-xtask) outlines an approach for
automating tasks in a Rust project. It involves using cargo's aliasing
feature to allow us to run commands like `cargo xtask <some task>` to
perform adhoc tasks via a Rust binary that we might otherwise need a
shell script for.

In this PR we add support for a `cargo xtask install-tools` command that
will replace the `install-mdbook.sh` script and install the dependent
tools. We can potentially extend it to support for other tasks e.g.
`cargo xtask fmt`.

---------

Co-authored-by: Eric Githinji <egithinji@google.com>

.github/workflows/install-mdbook/action.yml

@@ -6,7 +6,7 @@ runs:
   using: composite
   steps:
     - name: Install mdbook
-      run: ./install-mdbook.sh
+      run: cargo xtask install-tools
       shell: bash
 
     - name: Install dependencies for mdbook-pandoc