feature: Support for multiple endpoints by chainID

[lucasmenendez]

With the current logic of multiple web3 providers support, if there is a web3 providers on the same chain declared twice, the last one will be the only stored.

A potential new feature that we might add in the future is to have a pool of web3 providers that automatically connect to the next if the current fail. You might take some ideas or code from the old ethservice queue we had when the Oracle was connecting to Ethereum (https://github.com/vocdoni/vocdoni-node/blob/release-azeno/service/ethevents.go#L109) and (https://github.com/vocdoni/vocdoni-node/blob/release-azeno/ethereum/handler/ethereumHandler.go#L90).

In any case, this is probably out of scope on this PR. However, we might start thinking about it; thus providers might be a map [chainID] -> Web3ProviderHandler

Where Web3ProviderHandle has a list of methods, such as:

ChainID() int64
AddProvider(string)
NextProvider() ethClient
GetProvider() ethClient
String() string

The web3ProvidersHandler could make the Connect and check if the RPC endpoint actually works and is synced before returning it.

[TBD]