Add certificate chain validation to MutualSSLAuthenticator handler

[SavinduDimal]

Purpose

• Resolves Cannot use custom root CA with Mutual SSL and invoke APIs with a signed certificate api-manager#873

Description

• In the current implementation of MutualSSLAuthenticator handler client certificate should exist in the truststore to a successful validation
• This PR introduces certificate chain validation to resolve the above limitation
• Following configuration is introduced to the deployment.toml to enable certificate chain validation

[apimgt.mutual_ssl]
enable_certificate_chain_validation = true

[codecov]

Codecov Report

Attention: Patch coverage is 12.24490% with 129 lines in your changes are missing coverage. Please review.

Project coverage is 47.34%. Comparing base (062820a) to head (60f0dab).
Report is 35 commits behind head on master.

Files	Patch %	Lines
...security/authenticator/MutualSSLAuthenticator.java	7.44%	85 Missing and 2 partials ⚠️
...org/wso2/carbon/apimgt/gateway/handlers/Utils.java	32.25%	18 Missing and 3 partials ⚠️
...n/apimgt/impl/utils/GatewayCertificateMgtUtil.java	0.00%	21 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #12289      +/-   ##
============================================
+ Coverage     47.29%   47.34%   +0.04%     
- Complexity     4123     4145      +22     
============================================
  Files          1840     1841       +1     
  Lines        136407   136577     +170     
  Branches      19734    19776      +42     
============================================
+ Hits          64517    64664     +147     
- Misses        64281    64285       +4     
- Partials       7609     7628      +19     

Flag	Coverage Δ
integration_tests	42.08% <12.24%> (+0.08%)	⬆️
unit_tests	20.40% <3.40%> (-0.04%)	⬇️

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