Jedd/cxpla 154 enforce nullability analysers in objects and use required

[JR-Morgan]

Enables for the Speckle.Objects project the remaining nullability analysers for ensuring correct nullability and use of required keyword of our object models.

CS8618 - Non-nullable variable must contain a non-null value when exiting constructor.

• Enable CS8618 for Objects, still supressed for SDK
• Added required keyword for Pontcloud and Surface
• Surface class has some complex behaviour around the flattening the control point data for efficient serilization. For now I've had to expose a parametered constructor in addition to required properties, we could simplyfy this by re-structuring the class a little.

[linear]

CXPLA-154 Enforce nullability analysers in Objects and use `Required` keyword

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 20 lines in your changes missing coverage. Please review.

Project coverage is 69.13%. Comparing base (deddb21) to head (912971d).

Files with missing lines	Patch %	Lines
src/Speckle.Objects/Geometry/Surface.cs	0.00%	20 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##              dev     #202       +/-   ##
===========================================
+ Coverage   57.29%   69.13%   +11.84%     
===========================================
  Files         243      259       +16     
  Lines       10020    10742      +722     
  Branches     1090     1091        +1     
===========================================
+ Hits         5741     7427     +1686     
+ Misses       4022     2968     -1054     
- Partials      257      347       +90     

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