CloudFormation models for some IoT objects

[jsuchenia]

I'd like to contribute CloudFormation models for AWS IoT Core service.
Initial models for:

• AWS::IoT::Thing
• AWS::IoT::ThingType
• AWS::IoT::Policy

[codecov]

Codecov Report

Attention: Patch coverage is 91.53846% with 11 lines in your changes missing coverage. Please review.

Project coverage is 94.55%. Comparing base (5f4c48b) to head (c10a308).

Files with missing lines	Patch %	Lines
moto/iot/models.py	91.47%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8277      +/-   ##
==========================================
- Coverage   94.56%   94.55%   -0.01%     
==========================================
  Files        1158     1158              
  Lines      100606   100732     +126     
==========================================
+ Hits        95134    95249     +115     
- Misses       5472     5483      +11     

Flag	Coverage Δ
servertests	28.87% <37.69%> (+<0.01%)	⬆️
unittests	94.52% <91.53%> (-0.01%)	⬇️

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[bblommers]

LGTM - thank you for adding support for these models @jsuchenia!