feat: add the authentication_key for BGP MD5 authentication in directlink service

[ajay-malhotra1]

PR summary

This PR addresses the following changes:

1. Add the option to add the authentication_key while approving provider created gateways
2. Add the authentication_key in the response while fetching gateways

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 #26 (66ae69c) into master (1cac1bb) will increase coverage by 0.01%.
The diff coverage is 87.30%.

@@            Coverage Diff             @@
##           master      #26      +/-   ##
==========================================
+ Coverage   89.42%   89.43%   +0.01%     
==========================================
  Files          37       37              
  Lines       22276    22339      +63     
==========================================
+ Hits        19920    19980      +60     
- Misses       2356     2359       +3     

Impacted Files	Coverage Δ
ibm_cloud_networking_services/direct_link_v1.py	89.94% <87.30%> (-0.09%)	⬇️
...oud_networking_services/direct_link_provider_v2.py	88.66% <0.00%> (+0.71%)	⬆️

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[ajay-malhotra1]

🎉 This PR is included in version 0.6.0 🎉

The release is available on GitHub release

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