Support fetching pending nonce

[yihuang]

Proposal: Support fetching pending nonce

Current behavior: only support fetching from current deliver state

Desired behavior: ethereum api supports query on latest/pending/historical block height state.

Use case:

Fetching pending nonce when sending a new transaction is a common pattern in web3 development.

Related code in go-ethereum: https://github.com/ethereum/go-ethereum/blob/master/internal/ethapi/api.go#L1475

I think it's tricky to implement, but I create this issue nonetheless.

[fedekunze]

See https://github.com/cosmos/ethermint/blob/3596d535e1e32113604c122ffbdb0c68fac35ba3/rpc/namespaces/eth/api.go#L1101-L1147