build: add watch/compile tasks for CLI (#182344)

* build: add watch/compile tasks for CLI

I spent time this morning working on the 'developer experience' of the
CLI in vscode, mainly getting the CLI to cross-compile chasing our
initial idea of having it auto-build in a devcontainer.

After some effort and disabling tunnels connections (to avoid having to
pull in OpenSSL which is a huge pain to cross compile), I was able to
get it to cross-compile from Linux to Windows, using the mingw linker.
I could probably figure out how to get macOS working as well with more
effort. However, I'm not a big fan of this, effectively it's one more
'platform' of build we need to support and test.

I think a better approach is downloading the latest compiled CLI from
the update server instead, as needed. That's what this PR does. It just
places the CLI where it _would_ normally get compiled to by cargo; so
far we don't need to do anything special outside of that.
A notice is shown to users if this fallback happens.

* update from review