internal/ethapi: add optional parameter blockNrOrHash to estimateGas (e…

…thereum#21545)

This allows users to estimate gas on top of arbitrary blocks as well as pending and latest.
Tracing on pending is useful for most users as it takes into account the current txpool while
tracing on latest might be useful for users that have little to know knowledge of the current
transactions in the network.

If blockNrOrHash is not specified, estimateGas defaults to pending