Update amounts from u32 to u64 in create requests

[ok300]

Some amount fields in swap create requests were u32, while other relevant fields (fees, min/max in pair limits) were u64.

This meant conversion or typecasting was often necessary when interacting between the two.

To simplify this in callers, this PR updates the u32 amount fields in Swap Create requests to u64.

[i5hi]

Thanks @ok300