Expose totalDifficulty in go-ethereum client

[zimbabao]

This PR contains API to expose totalDifficulty in go-ethereum client.
APIs like JSON-RPC, web3.js all expose totalDifficulty, but somehow go-ethereum was not exposing it.
Block object was populated with totalDifficulty as a private field but was not used anywhere.

[fjl]

I would rather remove the td field. This field will lose its meaning very soon when the PoS transition happens. I'm sorry that we cannot provide this information when loading blocks via RPC, but it is arguably a niche use case.

If you must get the totalDifficulty via RPC, you can always do this:

type rpcBlock struct{
    Number *hexutil.Big `json:"number"`
    TD     *hexutil.Big `json:"totalDifficulty"`
}

c, err := rpc.Dial("<url>")

var info rpcBlock
err := c.Call(&info, "eth_getBlockByNumber", "latest")
fmt.Println(info.TD)