Make basic auth providers isolated

[ldclakmal]

Purpose

$subject
Related to ballerina-platform/ballerina-library#584

Checklist

• Linked to an issue
• Updated the changelog
• Added tests

[codecov]

Codecov Report

Merging #209 (49fb8c0) into master (80ab270) will decrease coverage by 0.49%.
The diff coverage is 62.50%.

@@             Coverage Diff              @@
##             master     #209      +/-   ##
============================================
- Coverage     81.87%   81.37%   -0.50%     
  Complexity       94       94              
============================================
  Files            17       17              
  Lines           491      494       +3     
  Branches         80       82       +2     
============================================
  Hits            402      402              
- Misses           69       72       +3     
  Partials         20       20              

Impacted Files	Coverage Δ	Complexity Δ
...a/listener_ldap_user_store_basic_auth_provider.bal	82.85% <50.00%> (-5.03%)	0.00 <0.00> (ø)
auth-ballerina/client_basic_auth_provider.bal	90.00% <66.66%> (-10.00%)	0.00 <0.00> (ø)
...a/listener_file_user_store_basic_auth_provider.bal	95.45% <100.00%> (ø)	0.00 <0.00> (ø)

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