Refactor introspection api to consume shared implementation

[willmyrs]

Summary

As part of the effort to bring the Agent Introspection Server to the Fargate Agent, this change refactors the agent introspection server to consume the shared implementation in ecs-agent.

Implementation details

The http server logic for the introspection server has been moved to the shared ecs-agent library. The server depends on the AgentState interface which will be implemented in the ECS and Fargate agents. The AgentState implementations will supply the server with the data it needs to construct responses. This change refactors the ECS agent's introspection server to consume the shared introspection server.

Testing

Unit and integration tests

Existing unit and integration tests have been modified, and where applicable, additional tests have been added. Existing unit tests for introspection_server in agent/handlers/v1 were moved to an integration test file with the intent to:

1. Not lose test coverage and,
2. Verify that the behavior on the client-end is not affected by the refactored server implementation.

Manual tests

Outputs from the Agent introspection server were recorded from two test clusters: one using the current version of the ECS agent, and the other using a modified build with the shared introspection server implementation. E.g.,

/v1/metadata

Current implementation

curl -s http://localhost:51678/v1/metadata | python3 -mjson.tool
{
    "Cluster": "ecs",
    "ContainerInstanceArn": "arn:aws:ecs:us-west-2:337909766174:container-instance/ecs/9d800cebdaba4380a21de1ec90add214",
    "Version": "Amazon ECS Agent - v1.89.2 (*41d593c6)"
}

Shared implementation

curl -s http://localhost:51678/v1/metadata | python3 -mjson.tool
{
    "Cluster": "ecs-test",
    "ContainerInstanceArn": "arn:aws:ecs:us-west-2:337909766174:container-instance/ecs-test/0f8883751d474a62aefbc3e945ad857a",
    "Version": "Amazon ECS Agent - v1.89.3 (*c46c0f4d)"
}

New tests cover the changes: yes

Description for the changelog

Enhancement - Refactor introspection api to consume shared server in ecs-agent

Additional Information

Does this PR include breaking model changes? If so, Have you added transformation functions?
No

Does this PR include the addition of new environment variables in the README?
No

Licensing

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

[willmyrs]

This PR was split off from Move introspection server to ecs-agent into two pull requests:

1. Implement agent introspection server in ecs-agent #4482
2. Refactor introspection server to consume shared library (this PR)

[prateekchaudhry]

We are doing a lookup here, we should handle "not found" error here too imho

[prateekchaudhry]

I see that this is similar to current implementation, and in case ContainerMapByArn happens to return nil, the containers list will be empty