Change metadata file format to something WASM compatible

[andrewphilipsmith]

To be decided

One possible workaround for Urban-Analytics-Technology-Platform/popgetter#73 is to change the file format used for publishing the metadata files.

• The metadata files are always read in full.
• (One of) the advantage of parquet is that partial files can be read over http.
• Therefore, let us use a file format that can be read in WASM.
• Gzipped CSVs might be a good enough fit.

(There is already a workaround for reading parquet metrics themselves using DuckDB)

@sgreenbury and @andrewphilipsmith have discussed this.

This is a big change therefore we would like @stuartlynn opinion before implementing this.