[easy] [flyteagent] Add agent-service endpoint settings for flyte-core deployment

[Future-Outlier]

Tracking issue

#3936

Docs Link

developing agents: https://flyte--5208.org.readthedocs.build/en/5208/flyte_agents/developing_agents.html
testing agents in a local development cluster: https://flyte--5208.org.readthedocs.build/en/5208/flyte_agents/testing_agents_in_a_local_development_cluster.html

Why are the changes needed?

Let users set up agent service when using flyte-core deployment.

You can see the full dialogue here: https://flyte-org.slack.com/archives/CP2HDHKE1/p1712575079971829

What changes were proposed in this pull request?

add more information for users to override config map about agents.

defaultAgent:
    defaultTimeout: 10s
    endpoint: dns:///flyteagent.flyte.svc.cluster.local:8000
    insecure: true
    timeouts:
      CreateTask: 5s
      GetTask: 5s
      DeleteTask: 5s
      ExecuteTaskSync: 10s

How was this patch tested?

flyte-core deployment.

Screenshots

Check all the applicable boxes

• I updated the documentation accordingly.
• All new and existing tests passed.
• All commits are signed-off.

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