[proposal] Support LOAD DATA LOCAL INFILE for MySQL

[Jille]

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.

[Jille]

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

[Jille]

I have two PRs ready for this, and a documentation branch I'll send later.

#2220
#2219
Jille/sqlc@loaddata2...loaddata3