Allow JSON reader to sample across multiple files

[lnkuiper]

Following the discussion in #8865, I've changed schema sampling in the JSON reader.

The JSON reader follows the functionality of our other readers (CSV/Parquet) when it comes to reading multiple files, e.g., with a GLOB. sample_size rows of the first file are sampled to determine the schema, and the rest of the files are assumed to have the same schema as the first one. However, if union_by_name=true, then we sample sample_size rows from each input file to determine the schema.

More so than with CSV/Parquet, having many small JSON files is very common. If this is the case, our default sampling strategy is quite poor. If the first file does not contain all the fields, then we will throw an error at some point during parsing. We can set union_by_name=true, but this has performance implications and requires users to look at the documentation to figure out how to combine schemas.

In general, we'd like to be able to parse most inputs simply by doing SELECT * FROM '*.json';. Therefore, I've changed our schema sampling to always sample the first sample_size rows when union_by_name=false (default), even if this requires sampling from multiple files. For example, if we have sample_size=6, and we're querying two JSON files both with 5 rows each, we will sample the first 5 rows of the first file, and the first row of the second file to determine the schema. The default sample_size is 20480.

I hope this will make querying JSON more pleasant for our users.

[lnkuiper]

I also found that reading piped JSON in a recursive CTE didn't work, that's fixed too in this PR.

[Mytherin]

Thanks! LGTM