Harden explorer API to abuse

[jmrossy]

In response to abuse discussed here: https://discord.com/channels/935678348330434570/1244947293392736387/1244953319391236108

Options include authentication, rate limiting, DDoS protection services, or privatizing the API entirely.

[avious00]

ser is #9 a dupe? seems like a component of this issue

[avious00]

capturing that Cryptorank.io has asked for this, will need more requests to prioritize and reenable explorer api

[jmrossy]

@avious00 No #9 is not a dupe. There are two separate APIs. The Hasura GraphQL API, and the Explorer API (which under the covers uses hasura). Both are susceptible to abuse

[jmrossy]

Note, the API was removed here but it should be re-added: #77