This commit adds a "sql_addition" configure option for the PostgreSQL
writer, which allows, e.g., to insert an UPSERT option after the insert
call that is constructed by the plugin.

For a complete example, see the write-upsert testcase, which uses this
command to create a filter:

local filter: Log::Filter = [$name="postgres", $path="testtable", $writer=Log::WRITER_POSTGRESQL, $config=table(["dbname"]="testdb", ["port"]="7772", ["sql_addition"]="ON CONFLICT (i) DO UPDATE SET s=EXCLUDED.s")];