[cli] Update command "br scan" to allow network interface selection for

mDNS binding.

Within the OTBR practice application, the BR host will have several
interfaces. When users want to discover the border router using mDNS,
they will need to choose the specific interface.

This change introduces a new option for the CLI command `br scan`. User
can now specify a network interface using the syntax `br scan --netif
<network interface>. The chosen interface will be used for mDNS binding
through socket options.

src/app/cli/interpreter.cpp

@@ -112,6 +112,8 @@
 #define WARN_NETWORK_SELECTION_DROPPED "Network selection was dropped by the command"
 #define WARN_NETWORK_SELECTION_CHANGED "Network selection was changed by the command"
 
+#define SO_BINDTODEVICE 25
+
 #define COLOR_ALIAS_FAILED Console::Color::kYellow
 
 namespace ot {