Handle IEA EEI 2023-11 edition

[khaeru]

Follow-up from #162.

• That PR migrated code from message_data that handles an earlier (2020-12) edition of these data.
• The data is (a) proprietary and (b) provided in an idiosyncratic, slightly complicated format. For the earlier PR, it was not worth the effort to produce a "fuzzed" (random values, but identical structure) version of the data for testing purposes.
• For the 2023-11 edition of these data, the IEA website at https://www.iea.org/data-and-statistics/data-product/energy-efficiency-indicators provides a public "Demo and availability file".

To do:

• Adapt the code to fetch and read the "demo and availability file".
• Handle any differences between the 2020 and 2023 format.
• (Maybe) Handle intermediate versions, if they are available.