GCORE: support metadatas for controlling GeoDNS/Failover

[xddxdd]

Add metadata fields for configuring Gcore's GeoDNS and Healthcheck/Failover functionalities, as well as documentations for them.

Tested manually with my own config.

As a side note, I didn't add any automated tests because I'm not sure how we should do so for custom metadata fields. Should I add them as testgroups in integrationTest/integration_test.go?