feat: SDK Custom Resolver Location as Optional

[DwitikrDash]

PR summary

Fixes: <! -- link to issue -->

PR Checklist

Please make sure that your PR fulfills the following requirements:

• The commit message follows the Angular Commit Message Guidelines.
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• New tests
• Build/CI related changes
• Documentation content changes
• Other (please describe)

What is the current behavior?

What is the new behavior?

Does this PR introduce a breaking change?

• Yes
• No

Other information

[codecov]

Codecov Report

Merging #39 (483bed5) into master (b0045fe) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #39      +/-   ##
==========================================
- Coverage   88.73%   88.68%   -0.05%     
==========================================
  Files          37       37              
  Lines       24158    24158              
==========================================
- Hits        21437    21425      -12     
- Misses       2721     2733      +12     

Impacted Files	Coverage Δ
ibm_cloud_networking_services/dns_svcs_v1.py	92.65% <100.00%> (+0.33%)	⬆️
...oud_networking_services/direct_link_provider_v2.py	87.94% <0.00%> (-0.72%)	⬇️
...oud_networking_services/transit_gateway_apis_v1.py	90.39% <0.00%> (-0.43%)	⬇️
ibm_cloud_networking_services/direct_link_v1.py	89.62% <0.00%> (-0.28%)	⬇️
ibm_cloud_networking_services/zones_settings_v1.py	87.87% <0.00%> (-0.15%)	⬇️

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