Simplify the interface of conversion_call to allow a ConversionOptions object...

Simplify the interface of conversion_call to allow a ConversionOptions object that can be more easily extended. Currently any new argument needs changing a lot of call sites and there is redundancy in argument documentation.

Note: this does not modify the public symbols yet - it's not clear whether we want to complicate their interface. However we may want to use it in to_graph and to_code.
PiperOrigin-RevId: 213433379