feat(rds): add rds_cluster_protected_by_backup_plan check

[sansns]

Context

Add rds_cluster_protected_by_backup_plan check

Description

Add the cluster backup plan check to complement the instance backup plan check

Checklist

• Are there new checks included in this PR? Yes
  • If so, do we need to update permissions for the provider? Please review this carefully. No
• Review if the code is being covered by tests.
• Review if code is being documented following this specification https://github.com/google/styleguide/blob/gh-pages/pyguide.md#38-comments-and-docstrings
• Review if backport is needed.

License

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.86%. Comparing base (1b50fdb) to head (816702c).
Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5638      +/-   ##
==========================================
+ Coverage   89.80%   89.86%   +0.05%     
==========================================
  Files        1112     1114       +2     
  Lines       34490    34534      +44     
==========================================
+ Hits        30973    31033      +60     
+ Misses       3517     3501      -16     

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