feat(dump): support dumping only specified columns

[hiradp]

Summary

• Adds a new --columns flag to pscale database dump that allows specifying which columns to include when dumping specific tables
• Supports multiple tables with format: --columns 'users:id,name' --columns 'orders:id,total'
• The schema dump will include the complete schema.

Resolves #891

Testing

Added unit tests, but also manual verification against a live database.

$ go run cmd/pscale/main.go \
  database dump hp-msql-ps-10 main --columns "users:email" --columns "products:sku"

tarting to dump all tables from database hp-msql-ps-10 to folder /Users/hirad/code/planetscale/cli/pscale_dump_hp-msql-ps-10_main_20260120_193324
Dumping is finished! (elapsed time: 726.003583ms)

$ cat pscale_dump_hp-msql-ps-10_main_20260120_193324/hp-msql-ps-10.users.00001.sql
INSERT INTO `users`(`email`) VALUES
("bar@baz.test"),
("beta@gamma.test"),
("green@blue.test"),
("banana@cherry.test"),
("moon@star.test"),
("dog@bird.test"),
("south@east.test"),
("paper@scissors.test"),
("pong@ball.test"),
("world@test.test");

$ cat pscale_dump_hp-msql-ps-10_main_20260120_193324/hp-msql-ps-10.products.00001.sql
INSERT INTO `products`(`sku`) VALUES
("abc-123"),
("def-456"),
("ghi-789"),
("jkl-012"),
("mno-345"),
("pqr-678"),
("stu-901"),
("vwx-234"),
("yza-567"),
("bcd-890");