feat(agent): mount agent http-proxy credentials as secret

[francesco-furlan]

What this PR does / why we need it:

Mount agent http-proxy credentials as a secret in both agent DaemonSet and Deployment except when gke autopilot is enabled.

Checklist

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• Title of the PR starts with type and scope, (e.g. feat(agent,node-analyzer,sysdig-deploy):)
• Chart Version bumped for the respective charts
• Variables are documented in the README.md (or README.tpl in some charts)
• Check GithubAction checks (like lint) to avoid merge-check stoppers
• All test files are added in the tests folder of their respective chart and have a "_test" suffix

Check Contribution guidelines in README.md for more insight.