feat: GRE tunnel support for Transit Gateway

[kennburger]

PR summary

Support for new fields for creating a Transit Gateway connection of type gre_tunnel

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
• [X ] 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
• [X ] No

Other information

[codecov]

Codecov Report

Merging #27 (759a316) into master (c5b84e1) will increase coverage by 0.02%.
The diff coverage is 89.40%.

@@            Coverage Diff             @@
##           master      #27      +/-   ##
==========================================
+ Coverage   89.43%   89.46%   +0.02%     
==========================================
  Files          37       37              
  Lines       22339    22858     +519     
==========================================
+ Hits        19980    20450     +470     
- Misses       2359     2408      +49     

Impacted Files	Coverage Δ
...oud_networking_services/transit_gateway_apis_v1.py	90.80% <89.40%> (-0.19%)	⬇️

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[ajay-malhotra1]

🎉 This PR is included in version 0.7.0 🎉

The release is available on GitHub release

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