feat: NamedFields / NamedInsertFields / NamedUpdateFields

[sonnemusk]

Summary

Helpers to build named-query INSERT/UPDATE column lists from a struct or map without hand-writing every :name (issue #410).

type Client struct {
    ID     string `db:"id"`
    Name   string `db:"client_name"`
    Secret string `db:"client_secret"`
}

cols, vals, _ := sqlx.NamedInsertFields(client)
// cols: id, client_name, client_secret
// vals: :id, :client_name, :client_secret
db.NamedExec("INSERT INTO client ("+cols+") VALUES ("+vals+")", client)

set, _ := sqlx.NamedUpdateFields(client)
// set: id=:id, client_name=:client_name, client_secret=:client_secret
db.NamedExec("UPDATE client SET "+set+" WHERE id=:id", client)

Uses the same db tag / NameMapper rules as NamedExec and StructScan. Fields tagged db:"-" are omitted.

Does not generate full SQL or touch table metadata — only field name lists.

Addresses #410