Vendor SQLite amalgamation distribution. (#660)

This allows to ensure we're always using a known version of the SQLite lib
and allows pgcopydb code to use some "recent" features on all OS targets.

Such features include the RETURNING clause for DMLs, the FILTER clause for
aggregate function calls, and time formatting function or JSON functions.

See https://sqlite.org/amalgamation.html for details about that distribution
mode for SQLite.