Allow overwrite tlsExistingSecretName data key names

[avnerv]

Since we are using the k8s cert-manager to generate certificates, we face a limitation where modifying the data key names within the k8s secret, which is generated by the cert-manager, is not possible.
Considering this restriction, we need the option to overwrite the k8s secret data key names that you expect for in the helm chart:

  # Specifies an existing secret for tls-certificate must include:
  # - akeyless-api-cert.crt (base64)
  # - akeyless-api-cert.key (base64)

https://github.com/akeylesslabs/helm-charts/blob/main/charts/akeyless-api-gateway/values.yaml#L297-L300

Additionally, add the support of secret type kubernetes.io/tls