Controls where the stream of blocks will start.
The stream will start inclusively at the requested block num.
When not provided, starts at first streamable block of the chain. Not all
chain starts at the same block number, so you might get an higher block than
requested when using default value of 0.
Can be negative, will be resolved relative to the chain head block, assuming
a chain at head block #100, then using -50 as the value will start at block
#50. If it resolves before first streamable block of chain, we assume start
of chain.
If start_cursor is given, this value is ignored and the stream instead starts
immediately after the Block pointed by the opaque start_cursor value.