--resolver-address is accepted multiple times, but causes an error at runtime

[gautamg795]

The docs suggest that the --resolver-address flag can be provided multiple times, e.g.

   --resolver-address ADDRESS                  Make DNS requests to ADDRESS (IP:port).  Repeatable.

However, trying to start smokescreen with multiple resolvers fails:

{"level":"fatal","msg":"Could not create configuration: only one resolver address allowed, 2 provided","time":"2024-02-01T23:20:40.712120055Z"}

It appears support for multiple resolvers hasn't been implemented in the code yet: https://github.com/stripe/smokescreen/blob/8c0fa26edf63f35d5632ba7682d78ff07a306819/pkg/smokescreen/config.go#L208

Perhaps the docs should be updated first to reflect this, as I suspect the feature itself is nontrivial to implement?