Add selectFromDb hook for custom types

[partyplatter08-lab]

Summary

• add an optional selectFromDb(column, decoder, columnName) hook to customType() for PostgreSQL, MySQL, SQLite, SingleStore, and Gel custom columns
• route selected columns through the hook in normal selects, returning selections, and relational query JSON selections while preserving the existing fromDriver decoder path
• document the hook and add focused SQL-generation/decoder coverage across dialects, including relational query columns

Validation

• corepack pnpm install --frozen-lockfile
• corepack pnpm --filter drizzle-orm exec vitest run tests/custom-select-from-db.test.ts
• corepack pnpm exec dprint check docs/custom-types.lite.md drizzle-orm/src/column.ts drizzle-orm/src/utils.ts drizzle-orm/src/pg-core/columns/custom.ts drizzle-orm/src/mysql-core/columns/custom.ts drizzle-orm/src/sqlite-core/columns/custom.ts drizzle-orm/src/singlestore-core/columns/custom.ts drizzle-orm/src/gel-core/columns/custom.ts drizzle-orm/src/pg-core/dialect.ts drizzle-orm/src/mysql-core/dialect.ts drizzle-orm/src/sqlite-core/dialect.ts drizzle-orm/src/singlestore-core/dialect.ts drizzle-orm/src/gel-core/dialect.ts drizzle-orm/tests/custom-select-from-db.test.ts
• corepack pnpm --filter drizzle-orm exec tsc -p tests/tsconfig.json --noEmit
• corepack pnpm --filter drizzle-orm test
• corepack pnpm --filter drizzle-orm test:types
• PATH=/tmp/corepack-bin:$PATH corepack pnpm --filter drizzle-orm build

/claim #1083