Allow provinding own sqlite3.c

[Christiaan676]

The bundled feature uses sqlite3.c / sqlite3.h that is packaged in this crate. It would be nice if there was a feature flag + environment variable that allows supplying our own version of the sqlite-amalgamation that is than compiled by this crate.

For context, I'm currently working on a project that has the requirement that all libraries source code needs to be sourced from the official source. So repackaging the sqlite-amalgamation like is done in libsqlite3-sys is unfortunately not allowed. The workaround is now to compile SQLite before the rust compilation and use the SQLITE3_LIB_DIR and SQLITE3_INCLUDE_DIR environment variables. This would also allow user to update the sqlite version when needed.

I would be willing to make the required changes in the build.rs if a merge request to implement this would be accepted.