Web3ApiClient adding default config should extend interface implementations

[nerfZael]

When registering interface implementations

const client = new Web3ApiClient({
  interfaces: [
    { 
      uri: "uri1",
      implementations: [ "impl1" ]
    }
  ]
});

The expected behavior is to have the default implementations for that interface be added to the implementations array

client.getInterfaces();
/*
[
    { 
      uri: "uri1",
      implementations: [ "impl1" , "default1"]
    }
]
*/

The actual behavior is that the interface implementations pair is added.

client.getInterfaces();
/*
[
    { 
      uri: "uri1",
      implementations: [ "impl1" , "default1"]
    },
    { 
      uri: "uri1",
      implementations: [ "default1"]
    }
]
*/

Additionally, if the developer registers multiple interface implementations pairs for the same interface. They should be merged together:

const client = new Web3ApiClient({
  interfaces: [
    { 
      uri: "uri1",
      implementations: [ "impl1" ]
    },
    { 
      uri: "uri1",
      implementations: [ "impl2" ]
    }
  ]
});

client.getInterfaces();
/*
[
    { 
      uri: "uri1",
      implementations: [ "impl1" , "impl2"]
    }
]
*/

[nerfZael]

Fixed in #766