feat: compute Scenario results with a reference backend

[gdalle]

Fixes #834

@penelopeysm what do you think?

[penelopeysm]

Sorry I only got to this! I would be super pleased with this :)

[gdalle]

I thought you weren't using DITest in Turing?

[penelopeysm]

Yup, no DIT in Turing.jl itself (well, not as of the time of writing at least) — but it was this function in particular which made me think that this feature in DIT would be nice :)

[codecov]

Codecov Report

❌ Patch coverage is 98.03922% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 79.22%. Comparing base (08d5708) to head (dfd9b51).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...fferentiationInterfaceTest/src/scenarios/modify.jl	97.87%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (08d5708) and HEAD (dfd9b51). Click for more details.

HEAD has 7 uploads less than BASE

Flag	BASE (08d5708)	HEAD (dfd9b51)
DI	28	21

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #839       +/-   ##
===========================================
- Coverage   97.47%   79.22%   -18.26%     
===========================================
  Files         130      129        -1     
  Lines        7813     7812        -1     
===========================================
- Hits         7616     6189     -1427     
- Misses        197     1623     +1426     

Flag	Coverage Δ
DI	72.87% <ø> (-25.29%)	⬇️
DIT	95.71% <98.03%> (+0.05%)	⬆️

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