[Add] ExternalObject class to provide the capability to access external objects refernced in AttributeValueXHTML instances

[samatstariongroup]

Prerequisites

• [x] I have written a descriptive pull-request title
• [x] I have verified that there are no overlapping pull-requests open
• [x] I have verified that I am following the ReqIFSharp code style guidelines
• [x] I have provided test coverage for my change (where applicable)

Description

[Add] ExternalObject class to provide the capability to access external objects refernced in AttributeValueXHTML instances

[codecov[bot]]

Codecov Report

Merging #19 (7e5cfc4) into development (fcfbc9f) will increase coverage by 0.60%. The diff coverage is 97.52%.

:exclamation: Current head 7e5cfc4 differs from pull request most recent head b43c973. Consider uploading reports for the commit b43c973 to get more accurate results

@@               Coverage Diff               @@
##           development      #19      +/-   ##
===============================================
+ Coverage        87.83%   88.43%   +0.60%     
===============================================
  Files               78       80       +2     
  Lines             3830     3951     +121     
  Branches           341      349       +8     
===============================================
+ Hits              3364     3494     +130     
+ Misses             375      364      -11     
- Partials            91       93       +2     

Impacted Files	Coverage Δ
...ElementWithAttributes/SpecElementWithAttributes.cs	97.40% <ø> (ø)
ReqIFSharp/AttributeValue/AttributeValueXHTML.cs	81.41% <95.12%> (+7.80%)	:arrow_up:
ReqIFSharp/AttributeValue/ExternalObject.cs	97.29% <97.29%> (ø)
...s/AttributeValueTests/ExternalObjectTestFixture.cs	100.00% <100.00%> (ø)
ReqIFSharp.Tests/ReqIFDeSerializerTestFixture.cs	98.19% <100.00%> (+0.34%)	:arrow_up:
ReqIFSharp/ReqIFDeserializer.cs	83.90% <100.00%> (+2.82%)	:arrow_up:
ReqIFSharp/AttributeValue/AttributeValueBoolean.cs	86.79% <0.00%> (+9.43%)	:arrow_up:
.../AttributeDefinition/AttributeDefinitionBoolean.cs	75.00% <0.00%> (+11.66%)	:arrow_up:

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