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.
