[Models] Enable nullable (1 of x)

[iamcarbon]

This is the first of a few PR's to enable nullable on the models project and close #466

This PR:

• Matches the requiredness of various properties with the specifications. Some properties were annotated with required instead, to limit churn in our tests
• Makes a few more classes immutable

The next PR will turn enable nullable at the project level

Posting for early review / feedback.

[iamcarbon]

@joegoldman2 @abergs Review for review / feedback. We should get these final nullability changes in before the 4.0 release.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 77.41935% with 7 lines in your changes missing coverage. Please review.

Project coverage is 76.03%. Comparing base (f8471ac) to head (dd2d208).

Files with missing lines	Patch %	Lines
...bjects/AuthenticationExtensionsLargeBlobOutputs.cs	0.00%	3 Missing ⚠️
...Fido2.Models/Objects/CredentialPropertiesOutput.cs	0.00%	2 Missing ⚠️
Src/Fido2.Models/Objects/Version.cs	0.00%	2 Missing ⚠️

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #581   +/-   ##
=======================================
  Coverage   76.03%   76.03%           
=======================================
  Files         101      101           
  Lines        2600     2600           
  Branches      432      432           
=======================================
  Hits         1977     1977           
  Misses        513      513           
  Partials      110      110           

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[iamcarbon]

@abergs Quick ping on this. There's one more PR after this to flipping on enable on the project and updating the directives. That should be the last of the churn for Models in the 4.0 cycle.

[abergs]

Thank you @iamcarbon, looks good!