Add shared handler for network latency fault

[xxx0624]

Summary

This PR is to add a shared handler for network latency fault. Its implementation is not complete and will be worked on later.

Implementation details

Following the first handler, this PR is to -

• Add three methods in ecs-agent/tmds/handlers/fault/v1/handlers/handlers.go for starting/stopping/checking network latency fault. Each of them will 1. parse the request payload 2. check if it's valid and 3. respond to the client accordingly.
• Add a struct for the request of network latency fault in ecs-agent/tmds/handlers/fault/v1/types/types.go

Testing

New tests cover the changes:

Added unit test for StartNetworkLatency(), StopNetworkLatency(), and CheckNetworkLatency()

Description for the changelog

Feature: Add network latency handler

Additional Information

Does this PR include breaking model changes? If so, Have you added transformation functions?

Does this PR include the addition of new environment variables in the README?

Licensing

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.