Make OAuth2 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 #203 (1d8afec) into master (73341df) will decrease coverage by 0.92%.
The diff coverage is 83.92%.

@@             Coverage Diff              @@
##             master     #203      +/-   ##
============================================
- Coverage     89.05%   88.12%   -0.93%     
  Complexity       34       34              
============================================
  Files             8        8              
  Lines           548      556       +8     
  Branches        192      206      +14     
============================================
+ Hits            488      490       +2     
- Misses           52       58       +6     
  Partials          8        8              

Impacted Files	Coverage Δ	Complexity Δ
oauth2-ballerina/client_oauth2_provider.bal	90.03% <83.33%> (-2.15%)	0.00 <0.00> (ø)
oauth2-ballerina/listener_oauth2_provider.bal	90.15% <87.50%> (ø)	0.00 <0.00> (ø)

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