Splunk: Fix MAC address to display in proper STIX format

[thangaraj-ramesh]

Added transformer to handle any MAC value into Stix format. Added new unit test cases.
Added missed async await in api_client.

[delliott90]

Can you rename this PR to something that would make sense to the open source community? This is an open project and no one knows what CP4SCORE-24254 means.

[delliott90]

We actually generate our CHANGELOGS from the merge commits on the PRs https://github.com/opencybersecurityalliance/stix-shifter/blob/develop/CHANGELOG.md

[thangaraj-ramesh]

As suggested PR is renamed.

[codecov]

Codecov Report

Patch coverage: 78.57% and project coverage change: +0.01 🎉

Comparison is base (8795a37) 85.23% compared to head (03932e9) 85.24%.

❗ Current head 03932e9 differs from pull request most recent head 9a12077. Consider uploading reports for the commit 9a12077 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1386      +/-   ##
===========================================
+ Coverage    85.23%   85.24%   +0.01%     
===========================================
  Files          585      585              
  Lines        43758    43771      +13     
===========================================
+ Hits         37298    37314      +16     
+ Misses        6460     6457       -3     

Impacted Files	Coverage Δ
...ter_modules/splunk/stix_transmission/api_client.py	35.84% <14.28%> (ø)
...er_modules/splunk/stix_translation/transformers.py	93.54% <100.00%> (+3.54%)	⬆️
...tests/stix_translation/test_splunk_json_to_stix.py	99.52% <100.00%> (+0.01%)	⬆️

... and 4 files with indirect coverage changes

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