Add getter for data for `Metadata` and `FlatVar`

In ClimaCalibrate, there is code that are written like this:

https://github.com/CliMA/ClimaCalibrate.jl/blob/31cda9c43db6352772229ebf8e059ed1ff67916a/ext/ClimaAnalysisExt.jl#L61C13-L67C23

Add a function like `get_data` would improve the formatting of the code.

Start by locating the definitions of Metadata and FlatVar and inspect the ClimaCalibrateExt.jl usage linked in the issue. Add a get_data entry point that provides the underlying data for both types, then verify that the referenced formatting code can use it and run the relevant existing tests.

Written by the indexing model from the issue text.