Add tests for API policies with secret attributes

[Piumal1999]

Purpose

Purpose of this PR is to add the integration tests for the API policies with secret type policy attributes
which is the feature developed under wso2/api-manager#3797

Approach

• Added a new policy with secret type policy attributes
• Added tests for create, attach, retrieval, and update scenarios

Added the following tests:

testAddAPISpecificOperationPolicyWithSecrets (Add API specific operation policy with secret attributes)
testAPIBeforeAttachingPolicyWithSecretAttributes (Test API before attaching secret policy)
testAPIAfterAttachingPolicyWithSecretAttributes (Test API after attaching policy with secret attributes)
testRetrievePolicyWithSecretAttributes (Test secret policy attribute retrieval)
testUpdatePolicyWithSecretAttributes (Test updating secret policy attributes with preserve option)
testVersionCreationWithPolicyWithSecretAttributes (Test version creation with secret policy attributes)
testDeleteAPISpecificOperationPolicyWithSecrets (Delete API specific operation policy with secret attributes)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 10.36%. Comparing base (7bbc46d) to head (9f38194).
⚠️ Report is 11 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #13797      +/-   ##
============================================
- Coverage     12.77%   10.36%   -2.41%     
+ Complexity      728      625     -103     
============================================
  Files           295      295              
  Lines         14089    14123      +34     
  Branches       1504     1508       +4     
============================================
- Hits           1800     1464     -336     
- Misses        12263    12641     +378     
+ Partials         26       18       -8     

Flag	Coverage Δ
integration_tests	10.36% <ø> (-2.41%)	⬇️

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