feat: Improving the env vars management in the graphql-server

[imrajdas]

Signed-off-by: Raj Babu Das mail.rajdas@gmail.com

Proposed changes

• os.GetEnv has been used a lot of times in the codebase. Moving all the envs to EnvConfig makes it easy to identify the vars in one place and easily manageable.
• Move the "cluster/agent" related functions to pkg/cluster.

Types of changes

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• New feature (non-breaking change which adds functionality)
• Bugfix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices applies)

Checklist

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• I have read the CONTRIBUTING doc
• I have signed the commit for DCO to be passed.
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have added necessary documentation (if appropriate)

Dependency

• Please add the links to the dependent PR need to be merged before this (if any).

Special notes for your reviewer:

[Jonsy13]

Apart from 1 comment, rest looks good!