covidregionaldata: Add additional subregional datasets

[mjrolland]

covidregionaldata is an R package that is used to provide subregional data for Rt estimates produced each day (https://epiforecasts.io/covid/). Due to developer time being limited, and the majority of the current focus being on EpiNow2, covidregionaldata is lacking on going support.

Adding additional datasets or linking to other packages that provide them would enable our group to potentially more easily provide subregional Rt estimates for areas covered by this data.

• Review current datasets
• Review R package space to see if alternative packages provide this and if they are reliable. If yes provide a wrapper to pull from these packages.
• Add subregional data based on the package SMG.

[impact: This package is used to support subregional reproduction number estimates that are used routinely by organisations such as the WHO and multiple governments. Providing reliable data is crucial to this pipeline. ]
[originally proposed by @seabbs]
[suggested repo: https://github.com/epiforecasts/covidregionaldata | @seabbs]
[additional notes: ]