Support Custom Endpoints for AWS Services

[jihunseol]

What would you like to be added:
I propose adding support for configuring custom endpoints for AWS services (STS, EC2, and CloudWatch Logs) in the aws-network-policy-agent. This enhancement would allow users to specify custom endpoints through configuration flags or environment variables, enabling the agent to interact with AWS-compatible services or operate in specialized environments.

Why is this needed:
Supporting custom endpoints is essential for flexibility and adaptability in various deployment scenarios:

• AWS-Compatible Services: In environments where AWS-compatible services are provided by third parties or on-premises solutions, the ability to configure custom endpoints allows the agent to interact seamlessly with these services.

• Air-Gapped and Private Networks: For deployments in air-gapped networks or isolated regions (such as AWS GovCloud or China regions), standard AWS endpoints may not be accessible. Custom endpoint support ensures the agent can function correctly in these restricted environments.

• Testing and Development: Allowing custom endpoints facilitates local testing and development using service emulators or mocks, improving the development workflow and enabling thorough testing without incurring AWS costs.

Implementing this feature would enhance the versatility and usability of the aws-network-policy-agent, making it more robust and suitable for a wider range of use cases.