Support for Basic authentication for networks

[ferjep]

Is your feature request related to a problem? Please describe. For nodes behind http auth it's a problem. We can't provide user and password to defaultNodeUrl or rpcUrl. Unless we do it within the url

Describe the solution you'd like Web3js provides the possibility to add headers to the provider. Maybe headers support is more useful for other cases. That'd be my way to go

Describe alternatives you've considered Include the user and password in the URL itself http://user:password@url.

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