refactor: create common package for holding STUN input (#631)

We want stunreachability to use the same STUN servers used by
snowflake, so let's start by making a common package holding the
servers. Let's also use this new package in Snowflake.

We're currently not using this package in stunreachability, but
I am going to apply this as a subsequent diff.

Reference issue: https://github.com/ooni/probe/issues/1814. This
issue is a bit complex to address in a single PR, so we are going
to proceed incremntally.

This diff was extracted from https://github.com/ooni/probe-cli/pull/539.

internal/ptx/snowflake.go

@@ -5,6 +5,7 @@ import (
 	"net"
 
 	sflib "git.torproject.org/pluggable-transports/snowflake.git/client/lib"
+	"github.com/ooni/probe-cli/v3/internal/stuninput"
 )
 
 // SnowflakeDialer is a dialer for snowflake. When optional fields are
@@ -115,20 +116,7 @@ func (d *SnowflakeDialer) iceAddresses() []string {
 	if len(d.ICEAddresses) > 0 {
 		return d.ICEAddresses
 	}
-	return []string{
-		"stun:stun.voip.blackberry.com:3478",
-		"stun:stun.altar.com.pl:3478",
-		"stun:stun.antisip.com:3478",
-		"stun:stun.bluesip.net:3478",
-		"stun:stun.dus.net:3478",
-		"stun:stun.epygi.com:3478",
-		"stun:stun.sonetel.com:3478",
-		"stun:stun.sonetel.net:3478",
-		"stun:stun.stunprotocol.org:3478",
-		"stun:stun.uls.co.za:3478",
-		"stun:stun.voipgate.com:3478",
-		"stun:stun.voys.nl:3478",
-	}
+	return stuninput.AsSnowflakeInput()
 }
 
 // maxSnowflakes returns the number of snowflakes to collect.