CNS: add "enabled" boolean to chain struct

[gamarin2]

When adding a new chain to the backend, we must go through a certain number of steps: #12

An entry in the CNS will be created for a chain before it is ready to be used in production and queried from the front-end. Therefore, I propose we add a enabled boolean to the chain struct. enabled will be manually set to true only after all the steps in the process have been properly completed.

Endpoints should only return value of enabled chains. For example, the /balances response should only contain balances of enabled chains.

cc @gsora @clockworkgr @lukitsbrian

[gsora]

@gamarin2 the /chains endpoint should list non-enabled chains?

/chain/{chainName} base path should return 404 for chains that are not enabled?

[gamarin2]

I think we should work under the assumption that no info about disabled chain should be returned to the front-end

[gsora]

Implemented in 9295877.