[proposal] Support LOAD DATA LOCAL INFILE for MySQL

https://github.com/go-sql-driver/mysql#load-data-local-infile-support supports LOAD DATA LOCAL INFILE with an io.Reader.

We could implement the :copyfrom query annotation to use LOAD DATA LOCAL INFILE when using go-sql-driver. The generated code would call https://pkg.go.dev/github.com/go-sql-driver/mysql#RegisterReaderHandler with a new reader for this call (and deregister it afterwards).

I propose to keep the same :copyfrom name and semantics, even though that's technically incorrect for MySQL.

I've looked at the three MySQL drivers listed on https://github.com/golang/go/wiki/SQLDrivers and the other two don't seem to support LOAD DATA [LOCAL] INFILE, though they might support it in the future.

We'd probably want to add a setting where people can indicate which MySQL driver they're using so we can generate the code using the relevant package.

Some more thoughts:

We need to encode the sqlc rows to TSV (or another CSV-like format) that is accepted by LOAD DATA INFILE. I propose to create an external library for that and let the generated code depend on it. (It felt like too much to generate inline.)
We need to configure sqlc which mysql driver is used, because database/sql doesn't natively support LOAD DATA INFILE. I think sql_package is the right place to put that. Passing go-sql-driver/mysql would behave mostly the same as passing database/sql, except that it allows :copyfrom support

Allow sql_package: "go-sql-driver/mysql"