Support custom MySQL dial network

[rebelice]

Summary

• Add optional custom connection hooks to mysql.ConnectionConfig so embedded callers can route gh-ost through a caller-managed SSH tunnel.
• Support Network for go-sql-driver/mysql DSNs instead of always generating @tcp(host:port).
• Support Dialer for go-mysql-org/go-mysql binlog streamer connections, which do not use database/sql DSNs.
• Preserve both custom hooks when duplicating connection configs, so inspector, applier, streamer, and replica/throttle connections continue to use the same dial path.
• Add unit coverage for custom-network DSN generation and duplication behavior.

Usage

Callers that need to route gh-ost connections through an SSH tunnel can register a custom dialer with go-sql-driver/mysql, then assign the registered network name and a go-mysql dialer to the gh-ost connection config:

network := "mysql-tcp-" + uuid.NewString()[:8]
mysql.RegisterDialContext(network, func(ctx context.Context, addr string) (net.Conn, error) {
    return sshClient.Dial("tcp", addr)
})
defer mysql.DeregisterDialContext(network)

dbConfig := migrationContext.InspectorConnectionConfig
dbConfig.Network = network
dbConfig.Dialer = func(ctx context.Context, network, addr string) (net.Conn, error) {
    return sshClient.Dial(network, addr)
}

After this, gh-ost SQL connections build DSNs like:

user:password@mysql-tcp-12345678(db.internal:3306)/dbname?...

This follows the extension model provided by go-sql-driver/mysql: callers register a custom network via RegisterDialContext, then use that network in the DSN as network(address). gh-ost2 also needs the explicit Dialer because binlog streaming uses go-mysql-org/go-mysql, not database/sql.

With this approach, gh-ost2 remains responsible for connection configuration, while the embedding application owns SSH authentication, secret handling, host-key policy, lifecycle, and cleanup.

Test Plan

• go test ./go/mysql ./go/binlog ./go/base
• git diff --check

Note: I also tried go test ./go/..., but the full suite did not exit in this local environment, so this PR validates the changed packages plus adjacent pure unit packages.