Allow attribute values to come from resource metadata

[dhutchison]

Fixes #394

I had started going down a bit of a rabbit hole of how hooks could be introduced so that attribute values could be more accurately generated, and that the library user could extend this instead of waiting on a library update to support new types. That seems quite complicated however for how infrequently I expect it to be required/used. The end of that approach was going to be that the hook updated the Metadata anyway (so calling GetAtt a second time for the same resource and attribute name would get a cached value so it was deterministic) - so if that idea comes back around, it will slot in to this fix.

This change allows what GetAtt returns to be set by the resource Metadata in the template. When unit testing there are no real AWS resources created, and cloud-radar does not attempt to realistically generate attribute values - a string is always returned. This works good enough most of the time, but there are some cases where if you are attempting to apply intrinsic functions against the attribute value it needs to be more correct.

Attribute values can be defined like this:

Resources:
  MediaPackageV2Channel:
    Type: AWS::MediaPackageV2::Channel
    Metadata:
      Cloud-Radar:
        attribute-values:
        # When unit testing there are no real AWS resources created, and cloud-radar
        # does not attempt to realistically generate attribute values - a string is always
        # returned. This works good enough most of the time, but there are some cases where
        # if you are attempting to apply intrinsic functions against the attribute value
        # it needs to be more correct.
        #
        # In this case, the attribute value is expected to be a List, not a string.
          IngestEndpointUrls:
            - http://one.example.com
            - http://two.example.com
    Properties:
      ChannelGroupName: dev_video_1
      ChannelName: !Sub ${AWS::StackName}-MediaPackageChannel

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.80%. Comparing base (f1b9d72) to head (ca0ac45).
Report is 93 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #399      +/-   ##
===========================================
- Coverage    95.00%   94.80%   -0.20%     
===========================================
  Files           11       12       +1     
  Lines          760      847      +87     
  Branches       155      159       +4     
===========================================
+ Hits           722      803      +81     
- Misses          21       24       +3     
- Partials        17       20       +3     

Flag	Coverage Δ
unit	94.80% <100.00%> (-0.20%)	⬇️

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