Fix backward incompatibility in OAuth client SSL context creation for proxy scenarios

[thisaltennakoon]

Fix wso2/api-manager#4239

Removes the backward incompatibility issue introduced by #2398

Overview

This PR refactors the SSL context creation in the OAuth client to improve trust store handling for proxy scenarios while maintaining backward compatibility with existing configurations, and improves trust store password management.

Key Changes

• Enhanced proxy SSL handling: Added proper SSL context creation for proxy scenarios using either a dynamic trust store (when TrustStoreConfigs is provided) or a static trust store fallback via system properties.
• Moved SSL context creation from a centralized method to specific methods for proxy and non-proxy scenarios.
• Fixed trust store password resolution issue: Previously, the trust store password was not being resolved correctly when HTTPS was used for proxy connections.