New endpoint /endpoints_simple

[f27d]

Current behavior

• Current /endpoints returns the JSON in the format used in NEO-MON with extra information

Expected behavior

• Add a new endpoint which returns the list of endpoints e.g. ["https://seed1.ngd.network:10332","https://seed1.cityofzion.io:443",..]
• These endpoints should have the same logic as /endpoints but just in a simpler format