ENS: option to use custom registry addresses

[sohkai]

Description

The current web3.eth.ens module only supports the official ENS registries on various networks, but there are some custom ENS deployments (especially on testnets) that users may want to resolve from.

Expected behavior

Upon creating the ENS module, the user could pass an option to specify the registry address (e.g. see ethereum-ens's options).

Actual behavior

The ENS module self-selects a registry address based on the current network, without giving the user a chance to override it.

Versions

• web3.js: v1.2.1 and v2

It would be ideal for this to be included in both v1 and v2, but if we'd like to avoid new features to v1, it would make sense to add it only to v2.