deployment: expose GCP secret manager option

[migmartri]

At #124 we added support to GCP secret manager as credentials backend. This option is not yet exposed in the Helm Chart

chainloop/deployment/chainloop/values.yaml

Line 32 in 707a62c

secretsBackend:

This issue is about adding support to configure GCP secret manager at deployment. This requires

• Add option in Helm Chart
• Docs
  • Readme (remember, we use helm generator https://github.com/bitnami-labs/readme-generator-for-helm to document options)
  • Chainloop docs

refs #108