feat: add option to bind outgoing connections to a specific interface

This adds a new `interface` configuration option to `doh-client` that allows users to specify a network interface for all outgoing DNS queries (including bootstrap and passthrough traffic).

doh-client/config/config.go

@@ -50,6 +50,7 @@ type others struct {
 	Bootstrap             []string `toml:"bootstrap"`
 	Passthrough           []string `toml:"passthrough"`
 	Timeout               uint     `toml:"timeout"`
+	Interface             string   `toml:"interface"`
 	NoCookies             bool     `toml:"no_cookies"`
 	NoECS                 bool     `toml:"no_ecs"`
 	NoIPv6                bool     `toml:"no_ipv6"`