Embed DuckDB extensions in the Rill binary

At present we download the binaries when starting rill, which can fail if duckDB CDN is having issues. 
```
HTTP Error: Failed to download extension "json" at URL "http://nightly-extensions.duckdb.org/v0.9.2/osx_arm64/json.duckdb_extension.gz" Extension "json" is an existing extension.
```

We would like to avoid downloading the extensions at runtime. 