feat(operator): allow the use of existing secrets

[aresfkonrad]

What changes are being made and why?

---

Closes #55
This feature allows the use of external secrets for the Kestra operator

How the changes have been QAed?

---

Setup Instructions

To enable the feature, the standard kestra values.yaml can be used. The following changes have to be made:

# To enable Basic Auth / apiKey auth via plain text fields
operator:
  enabled: true
  apiKey: "somerandomapikey"
  basicAuth: "user:secretpassword"

# To enable Basic Auth / apiKey auth via existingSecrets
operator:
  enabled: true
  apiKeyExistingSecretName: "kestra-apikey"
  apiKeyExistingSecretKey: "apiKey"
  basicAuthExistingSecretName: "kestra-basicauth"
  basicAuthExistingSecretKey: "basicAuth"
extraManifests:
  - apiVersion: v1
    data:
      basicAuth: aGFsbG86cGFzc3dvcmQ=
    kind: Secret
    metadata:
      name: kestra-basicauth
      namespace: kestra
  - apiVersion: v1
    data:
      apiKey: c29tZXJhbmRvbWtleXM=
    kind: Secret
    metadata:
      name: kestra-apikey
      namespace: kestra