Allow specifying allowed source address(es)

[Stebalien]

Currently, it's possible to specify listen addresses, but not source addresses for dials. It would be nice to be able to actually specify source addresses.

Specifically:

1. An initial implementation would allow specifying a single allowed source address. This option would need to be piped through to all transports and transports would need to be updated to obey it. Unfortunately, this would likely prevent, e.g., loopback dials. This trivial from a purely technical perspective, but we don't currently have a nice way to reliably pipe this information through from go-libp2p to the individual transports.
2. After the initial implementation, support for multiple allowed source addresses. Unfortunately, this is significantly more complicated to implement.

At the moment, the best solution here is to sandbox your application (i.e., use a network namespace) so libp2p simply doesn't have access to interfaces it's not allowed to use. Furthermore, restricting the application to a single interface/source address isn't something go-libp2p can actually enforce (i.e., some transports may ignore the setting and/or have bugs). Even if/when this feature is implemented, users should continue to sandbox libp2p if they don't want it to use/see some set of interfaces.

See #1063 for motivation.