Add support for NETWORK configuration parameter

[mollykarcher]

What problem does your feature solve?

See issue description where this was implemented in Horizon: stellar/go#4913

What would you like to see?

Both Horizon and Galexie support this configuration parameter network, which is optional and automatically sets some default values for several configuration values required by RPC. We'd like this parameter to be accepted by RPC in the same way it is for our other services.