Ability to select pre-orderable options without needing variants and pricing per option value

[hrithvik123]

Is your feature request related to a problem? Please describe.
We are trying to setup a store where products are custom built. To be more specific -- The products are car modifications such as bumpers, mudflaps, etc. Every product has a bunch of custom option choices you can choose. Depending on the options selected, a custom product is built. Every option value has a price associated with it.

Describe the solution you'd like
It would be nice if we could have custom built backorderable products without needing to define variants for option values. There is an extension called spree_flexi_variants that did this but I don't think it supports spree 4.6.

Describe alternatives you've considered
A way to do this currently would be to create variants for every possible combination, and assign prices to the variants. This could get very tedious once the if you have many options with many option values each. Also, creating so many variants makes the API painfully slow.

Additional context
For context about the kind of product I am dealing with, consider this Bushguard bumper with all these options. In this case there would be 768 variants. This makes the admin UI and the storefront API painfully slow.

Bushguard Bumper:

Vehicle Options:
            -Ford F150
	-Ford Excursion 
	-Chevy Silverado
	-Toyota 4Runner

Light Wing Options:
	-Standard 6” Rectangular Light Pocket (Speed Demon / Rigid)
	-Single Wide Dually 
	-Dual Dually
	-Standard 6” Pocket With Extra Dually Pocket
	-Single 6” SR Pocket
	-Dual 6” SR Pockets
	-Single B Pocket (6” Round)
	-Single S Pocket (4” Round)
	-Dual B Pockets
	-B & S Pockets
	-Dual S Pockets
	-Custom Light Pockets

Center Section Options:
	-Winch Ready
	-Standard Vent – Optional Mount Brackets For 20” Behind
	-Standard Mesh
	-Frenched 20” Light Bar

Lower Center Section Options:
	-Lower 6” SR Light Pockets
	-Integrated Receiver

Guard Area Options:
	-Extra Center Bar
	-Extra Headlight Protector Bars (Dual Thin Or Single)