Pull NeoFS endpoints from network map

[alexvanin]

For pilot installations, @realloc suggested having list of NeoFS endpoints from network map. But address field of node info should not be used, because it is going to be internal private address.

Instead, Node can define attribute PublicEndpoints with value grpc://public1:8080,grpcs://public1:8082. Consider addresses split by , or whitespace.

S3 Gateway makes request to the side chain and receives network map. If no public endpoint is defined, then use values from config.

[realloc]

Additionally, here is the point where we could use advanced logic like network/Geo proximity for choosing the most appropriate endpoints. In the future, of course.

[alexvanin]

I think @fyrchik did some tasks to split external and internal interfaces. Let's mention them here and look at it once again.

[alexvanin]

Looks very similar to nspcc-dev/neofs-sdk-go#297
Need to consider if both issues are valid. It seems like pool update can close this issue?