Make ImageReference strongly-typed and checked up-front

[Porges]

• Turn ImageReference into its own type so it is validated early on in request submission time, and we don't end up with malformed IDs, etc.
• Add in support for shared image galleries since that was easy enough to add while I'm doing this.
• Explicitly document which image sources are permitted and how to reference them with resource IDs.

This addresses/closes #1464 for the C# port. Also fixes #2927 which was recently reported.

---

Manually verified, I can create scalesets with:

• Latest image: /subscriptions/{}/resourceGroups/{}/providers/Microsoft.Compute/galleries/{}/images/{}
• Specific image: /subscriptions/{}/resourceGroups/{}/providers/Microsoft.Compute/galleries/{}/images/{}/versions/{}
• Marketplace image: Canonical:0000-com-ubuntu-server-focal:20_04-lts:latest
• VM Image /subscriptions/{}/resourceGroups/{}/providers/Microsoft.Compute/images/{}

Not manually verified:

• Latest shared image: /subscriptions/{}/resourceGroups/{}/providers/Microsoft.Compute/locations/{}/sharedGalleries/{}/images/{}
• Specific shared image: /subscriptions/{}/resourceGroups/{}/providers/Microsoft.Compute/locations/{}/sharedGalleries/{}/images/{}/versions/{}

[Porges]

Need to do some validation that shared images work as expected before merging.

[codecov-commenter]

Codecov Report

Merging #2369 (d195d5a) into main (6d5161c) will increase coverage by 0.15%.
The diff coverage is 41.98%.

@@            Coverage Diff             @@
##             main    #2369      +/-   ##
==========================================
+ Coverage   28.93%   29.09%   +0.15%     
==========================================
  Files         303      303              
  Lines       36133    36170      +37     
==========================================
+ Hits        10455    10523      +68     
+ Misses      25678    25647      -31     

Impacted Files	Coverage Δ
src/ApiService/ApiService/Functions/Scaleset.cs	39.28% <0.00%> (ø)
src/ApiService/ApiService/Program.cs	15.78% <ø> (+0.11%)	⬆️
...ApiService/ApiService/onefuzzlib/OnefuzzContext.cs	0.00% <ø> (ø)
...piService/ApiService/onefuzzlib/ProxyOperations.cs	0.00% <0.00%> (ø)
...piService/ApiService/onefuzzlib/ReproOperations.cs	10.81% <0.00%> (ø)
...c/ApiService/ApiService/onefuzzlib/VmOperations.cs	0.00% <0.00%> (ø)
...ApiService/ApiService/onefuzzlib/VmssOperations.cs	0.00% <0.00%> (ø)
...ervice/ApiService/onefuzzlib/ScalesetOperations.cs	2.87% <25.00%> (+0.19%)	⬆️
...ApiService/ApiService/onefuzzlib/ImageReference.cs	41.14% <41.14%> (ø)
...ApiService/ApiService/onefuzzlib/TaskOperations.cs	30.80% <50.00%> (+0.29%)	⬆️
... and 5 more

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