WIP: GCP Unify logic for credentials & support self signed cert for elastic (GONRG-539)

Description:

• Moved credentials classes into core-lib-gcp.
• Added a new 'ElasticClientHandlerGcp' to support self-signed certificates.
• Added a new env variables 'SECURITY_HTTPS_CERTIFICATE_TRUST' into app.yaml.
• Added a property 'security.https.certificate.trust' into application-*.properties.

How to test:

Turn on https with a custom self-signed cert for elastic. SECURITY_HTTPS_CERTIFICATE_TRUST = true

Changes include:

• Refactor (a non-breaking change that improves code maintainability).
• Bugfix (a non-breaking change that solves an issue).
• New feature (a non-breaking change that adds functionality).
• Breaking change (a change that is not backward-compatible and/or changes current functionality).

Changes in:

• GCP
• Azure
• AWS
• IBM

Dev Checklist:

• Added Unit Tests, wherever applicable.
• Updated the Readme, if applicable.
• Existing Tests pass
• Verified functionality locally
• Self Reviewed my code for formatting and complex business logic.

Other comments:

Any comments to approvers here