Support multiple web3 providers and networks

[p4u]

We might have specified via flag or env var a list of web3providers and networks. This way we would support multiple networks on the same service, enabling the option for creating multi-network censuses (which is a demanded feature).

The API needs to be sightly modified to support multiple networks.

• The available network list is returned by an API endpoint, we use the chainID as identifier
• When a token is added, user needs to specify the chainID
• When a token is retrieved, the backend returns the chainID of each token
• The same token can be added twice if the chainID is different

This is probably an epic task since requires changes on the SQL scheme and API definition.