refactor: Remove transitive dependency from swirlds-platform-core on consensus-hashgraph-impl

[poulok]

Description:
This PR eliminates the transitive dependency from swirlds-platform-core testFixtures on consensus-hashgraph-impl testFixture. The concern with this dependency is that classes like EventImpl could be leaked to a src/test module via the testFixtures.

SimpleGraph was split into two classes that implement a new interface, each parameterizing it with the type of event they provide. The one that provides PlatformEvent is exposed externally for other testFixtures and tests. The one that provides EventImpl is only exposed to the consensus-hashgraph-impl src and test module, not to any external modules. This ensures that classes such as EventImpl are not exposed outside of the module at all.

Related issue(s):

Fixes #23091

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[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ +0.00% (target: -1.00%)	✅ ∅

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (2a023e7)	91276	74848	82.00%
Head commit (74b53e8)	91276 (+0)	74852 (+4)	82.01% (+0.00%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#23389)	0	0	∅ (not applicable)

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@            Coverage Diff            @@
##               main   #23389   +/-   ##
=========================================
  Coverage     78.02%   78.02%           
+ Complexity    23413    23412    -1     
=========================================
  Files          2458     2458           
  Lines         91459    91459           
  Branches       9710     9710           
=========================================
+ Hits          71359    71364    +5     
+ Misses        16511    16507    -4     
+ Partials       3589     3588    -1     

see 15 files with indirect coverage changes

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