feat: add optional KUBECONFIG_BASE64 env to pass base64-encoded value

[shivanshs9]

Changes:

• Added a parseBase64 function to decode base64 secret value of KUBECONFIG_BASE64 env
• If the user hasn't set KUBECONFIG_FILE env, then try to decode KUBECONFIG_BASE64 env value.

Motivation:

• To make our workflow compliant with other github actions, we stored base64-encoded value of kubeconfig.
• Decoding it in a previous step and setting is as env or output of step doesn't work since GitHub Actions truncates multi-line strings.
• Decoding the secret from base64 and then encoding it to Github Actions compliant string didn't work since now Helm cannot parse the yaml.