Add possibility to use particular certificate for client during 2 way ssl connection

[dgrinenko]

JIRA: https://cdap.atlassian.net/browse/PLUGIN-1008

The issue:
In some cases, java struggling to auto-detect the proper client certificate for the 2-way SSL connection in the truststore, especially if the certificate contains custom constraints. It prevents plugin to establish a secure 2 way ssl connection

What need to be done:
In such cases - http plugin should provide possibility to ask user for specific certificate alias in the keystore, to use it later for client connection instead of trying to auto-detect it

What was done:

• Added new configuration item for the plugin - "Keystore Cert Alias"
• In case, if user set the alias for certificate, plugin would try to use it, instead of trying to auto-detect
• The feature is supported only for X.509 keystores/certificates

Unit tests passed:

Results :

Tests run: 41, Failures: 0, Errors: 0, Skipped: 0

[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  03:48 min
[INFO] Finished at: 2021-12-14T15:02:38+02:00
[INFO] ------------------------------------------------------------------------