Output extension to a sub-directory that respect the input mode/feature

[Y--]

Today, the build_loadable.sh scripts output every extension under loadable_extensions folder.

This PR outputs them under loadable_extensions/$MODE/$FEATURE where MODE and FEATURE are the parameters provided in input to select the source directory (eg. mode=relperf feature=mvp)

[carlopi]

I though about and solving this generally (= in duckdb Makefile), with having a postprocessing step where extensions are copied to build/release/extensions/${duckdb_version}/${platform}/${name}.duckdb_extension[.wasm], since that will also allow to use the folder as a local extension directory, basically SET custom_extension_repository='/path/to/folder';

Stuff might move more that direction, but for not this is still more clean.

Thanks.