Add inflation query

[noandrea]

The mint module does not provide a way to query the current inflation and mint schedule

an Inflation query

parameters

• height: the block height

returns:

• inflation ( range 0-1)
• epoch

eg:

InflationQueryRequest{
 height: 10,
}

response

InflationQueryResponse{
  Epoch: 1,
  Inflation: 1.0,
}