Add stripPrefix option to ExtractHooks

The hooks come packaged in a TAR inside custom_hooks directory. Currently ExtractHooks extracts this directory directly into the target folder. In the upcoming WAL implementation for custom hooks, the hooks will be stored in wal/hooks/<log_index>. That directory should contain the hooks directly and it's unnecessary to put them in a custom_hooks subdirectory in there.

This MR makes this possible by adding the stripPrefix option for ExtractHooks to remove the custom_hooks prefix from the extracted hooks.

Part of #4737 (closed)