feat: improve helm chart

[Demonsthere]

Hi there! A small PR with 2 features that i found useful :) :

• Add the possibility to specify a custom secret, this allows to use a pre-existing secret instead of supplying the data in plaintext within values
• Add custom pod labels to the deployment

[holms]

@nmldiegues maybe this could be approved? It's totally makes sense.

[5cat]

This pull request is essential, without it the chart is useless. Passing secrets as a plain text in a helm values file is insecure, this pull request provides a way to pass the secret name instead of the value, so I can use external-secret to set it and store it without having to hard code it in the helm values file. @eidam @chungthuang @adamchalmers

[gitguardian[bot]]

⚠️ GitGuardian has uncovered 1 secret following the scan of your pull request.

Please consider investigating the findings and remediating the incidents. Failure to do so may lead to compromising the associated services or software components.

🔎 Detected hardcoded secret in your pull request

GitGuardian id	Secret	Commit	Filename
-	Generic High Entropy Secret	763d4567f72be65adaabbd34e555c326e6f1d836	helm/cloudflare-tunnel/overrides.yaml	View secret

🛠 Guidelines to remediate hardcoded secrets

1. Understand the implications of revoking this secret by investigating where it is used in your code.
2. Replace and store your secret safely. Learn here the best practices.
3. Revoke and rotate this secret.
4. If possible, rewrite git history. Rewriting git history is not a trivial act. You might completely break other contributing developers' workflow and you risk accidentally deleting legitimate data.

To avoid such incidents in the future consider

• following these best practices for managing and storing secrets including API keys and other credentials
• install secret detection on pre-commit to catch secret before it leaves your machine and ease remediation.

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