Simple way to add metadata to geoparquet file

[Matthias Mohr]

I think it would be nice that if you have a Parquet file to just be able to provide the JSON metadata and write it to the file. Something like:

gpq add_metadata in.parquet out.geoparquet metadata.json
or
gpq convert in.parquet out.geoparquet --metadata="metadata.json"

In my use case I know the metadata upfront and just need to add the geoparquet metadata to the file.
Could also be a simple way to fix/override geoparuqet metadata in case of bugs such as #45