Make orderOptions.generateOrderCode injectable

[michaelbromley]

Is your feature request related to a problem? Please describe.
Currently you can define a custom function to generate order codes. One problem is that there is no check done to see if that code is unique, and there is no way to easily check this in the function itself because it is not injectable.

Describe the solution you'd like
Rather than the current function:

generateOrderCode?: (ctx: RequestContext) => string | Promise<string>;

we should accept an object which implements the InjectableStrategy interface which will allows the function itself to check for uniqueness.