Add certificate chain validation to MutualSSLAuthenticator handler

[SavinduDimal]

Purpose

• Resolves https://github.com/wso2/api-manager/issues/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[bot]]

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 :warning:
...org/wso2/carbon/apimgt/gateway/handlers/Utils.java	32.25%	18 Missing and 3 partials :warning:
...n/apimgt/impl/utils/GatewayCertificateMgtUtil.java	0.00%	21 Missing :warning:

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/wso2/carbon-apimgt/pull/12289/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=wso2) | Coverage Δ | | |---|---|---| | [integration_tests](https://app.codecov.io/gh/wso2/carbon-apimgt/pull/12289/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=wso2) | `42.08% <12.24%> (+0.08%)` | :arrow_up: | | [unit_tests](https://app.codecov.io/gh/wso2/carbon-apimgt/pull/12289/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=wso2) | `20.40% <3.40%> (-0.04%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=wso2#carryforward-flags-in-the-pull-request-comment) to find out more.

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