Add WebSocket subscription support

[joaop21]

This PR adds WebSocket subscription support, allowing users to subscribe to blockchain events such as new blocks, contract logs, and pending transactions.

Key changes:

• Implemented WebSocket subscription and unsubscription functionality
• Added support for multiple subscription types:
  • newHeads (new block headers)
  • logs (contract events with filtering)
  • newPendingTransactions
• Updated documentation with comprehensive WebSocket subscription examples
• Added new WebSocket-specific RPC methods to the documentation

Important Note:
Subscription events are delivered directly to the subscribing process's mailbox.
The process that calls subscribe/2 will receive the events as messages.

Example usage:

# Subscribe to new blocks
{:ok, sub_id} = Ethereumex.WebsocketClient.subscribe(:newHeads)

# Events will be delivered to the current process's mailbox
receive do
  %{
    "method" => "eth_subscription",
    "params" => %{
      "subscription" => ^sub_id,  # Pattern match on the subscription ID
      "result" => block_data
    }
  } -> 
    # Handle the new block data
    handle_block(block_data)
end

[ayrat555]

@joaop21 thank you!