Support events watching from specific contracts

[vbaranov]

Is your feature request related to a problem? Please describe.
It is not possible to specify contract addresses which events I would like to subscribe in log_subscriptions command. Am I right?

Describe the solution you'd like
Together with events specification, I would like to indicate in the config at which contracts those events happen.

Describe alternatives you've considered
Though it is possible to filter from the output in the log file, it would be awesome to have pre-filtering before output. Possibly, it is already implemented. However, I didn't find anything regarding this in the docs. If it exists, please point me out.

[rupurt]

💯 want to have this. Just haven't got around to it yet 😄

PR's welcome if you want to have a stab at it 😉

[rupurt]

@vbaranov @AwaitFuture

I've added an example of how I think the configuration DSL should work to support specific contract addresses https://github.com/fremantle-industries/slurp/blob/watch-events-from-specific-contract/config/dev.exs#L91

It should look something like this

config :slurp,
  log_subscriptions: %{
    "*" => %{
      {"Approval(address,address,uint256)",
       [
         "0x26CdfbAeD9F97ecBdc76cA5cC544f052Bd9Ac795",
         "0xFc979087305A826c2B2a0056cFAbA50aad3E6439"
       ]} => [
        %{
          enabled: true,
          struct: Examples.Erc20.Events.Approval,
          handler: {Examples.EventHandler, :handle_erc20_specific_address_approval, []},
          abi: [
            %{
              "anonymous" => false,
              "inputs" => [
                %{
                  "indexed" => true,
                  "name" => "owner",
                  "type" => "address"
                },
                %{
                  "indexed" => true,
                  "name" => "spender",
                  "type" => "address"
                },
                %{
                  "indexed" => false,
                  "name" => "value",
                  "type" => "uint256"
                }
              ],
              "name" => "Approval",
              "type" => "event"
            }
          ]
        }
      ],
    }
  }