Make a local server to host Azure data for local development

[sgreenbury]

As discussed with @penelopeysm and @stuartlynn, to aid local development it would be helpful to create a small crate binary that can:

• Download the Azure data to a local path to cache the popgetter data (if one is not available)
• Have the same API structure as Azure
• Support range requests (e.g. https://docs.rs/axum-range/latest/axum_range/)
• Ability to limit to one country to reduce the amount required to cache

Axum might be a good choice for this.

Idea: two commands download and serve