Change function where new config value is defined

[teaguesterling]

This should fix #84 by changing where the unsafe_enable_version_guessing config value is defined to the same function where other new functions are defined. This has been tested by the following:

~/duckdb_iceberg$ make test
...
cd
~/$ duckdb -unsigned
D FORCE INSTALL 'duckdb_iceberg/build/release/repository/v1.1.3/linux_amd64/iceberg.duckdb_extension';
D LOAD iceberg;
D .quit
cd ~/duckdb_iceberg/data/iceberg
~/duckdb_iceberg/data/iceberg$ duckdb -unsigned # note this is my system version of duckdb
D LOAD iceberg;
D SET unsafe_enable_version_guessing=true;
D SELECT COUNT() FROM iceberg_scan('lineitem_iceberg_no_hint');
┌──────────────┐
│ count_star() │
│    int64     │
├──────────────┤
│        51793 │
└──────────────┘