Fix stats for datasets bigger than 5Gb on first n parquets
Currently for datasets with lots of parquet files which were copied from the main branch statistics are not computed with `SplitWithTooBigParquetError` (example: `bigcode/starcoderdata`). While we can take first n parquet files instead, as it's done for `duckdb-index` runner. 

\+ also updated openapi.json with a change not from this PR (bool feature in stats)
\+ added check for "partial" field for /search and /filter endpoints in tests - or were they omitted intentionally?

Questions:
- I added `"partial"` field for statistics runner too but not sure how to update the cache. Previously datasets with partial exports (with `partial-` in directory) worked fine so I had to set `None` as default value to populate cache entry in migration because I cannot set either `True` or `False` (there are already both cases in cache).
- I noticed that `/search` Hub's don't have info about some keys that are returned in response (`num_rows_total`, `num_rows_per_page` and `partial`. Is it intentional? If not, I can add it. 
Fix stats for datasets bigger than 5Gb on first n parquets: Currently for datasets with lots of parquet files which were copied from the main branch statistics are not computed with `SplitWithTooBigParquetError` (example: `bigcode/starcoderdata`). While we can take first n parquet files instead, as it's done for `duckdb-index` runner. 

\+ also updated openapi.json with a change not from this PR (bool feature in stats)
\+ added check for "partial" field for /search and /filter endpoints in tests - or were they omitted intentionally?

Questions:
- I added `"partial"` field for statistics runner too but not sure how to update the cache. Previously datasets with partial exports (with `partial-` in directory) worked fine so I had to set `None` as default value to populate cache entry in migration because I cannot set either `True` or `False` (there are already both cases in cache).
- I noticed that `/search` Hub's don't have info about some keys that are returned in response (`num_rows_total`, `num_rows_per_page` and `partial`. Is it intentional? If not, I can add it. 