Keda vault service account token request

[BojanZelic]

TODO;

Still have to add documentation & helm chart

---

Allows users to authenticate to vault via a service account in the scaledObject's namespace;

ex:

apiVersion: keda.sh/v1alpha1
kind: TriggerAuthentication
metadata:
  name: my-trigger-auth
  namespace: mynamespace
spec:
...
  hashiCorpVault:
    address: {hashicorp-vault-address}
    credential:
      serviceAccountName: default

apiVersion: keda.sh/v1alpha1
kind: ScaledObject
metadata:
  name: my-scaled-object
  namespace: mynamespace
spec:
  triggers:
  - authenticationRef:
      name: my-trigger-auth
      kind: TriggerAuthentication
    metadata:
...

would use the JWT token from the default service account in the mynamespace namespace

This allows users to set more fine-grained permissions in vault.

Checklist

• I have verified that my change is according to the deprecations & breaking changes policy
• Tests have been added
• Changelog has been updated and is aligned with our changelog requirements
• A PR is opened to update our Helm chart Allow Keda to request SA tokens charts#728
• A PR is opened to update the documentation on document Vault authentication via cross-namespace service accounts keda-docs#1521
• Commits are signed with Developer Certificate of Origin (DCO - learn more)

Fixes # #6153

Relates to #

[semgrep-app]

Consider to use well-defined context

_{Ignore this finding from context-todo.}

[SpiritZhou]

Could you also add some e2e tests?

[SpiritZhou]

Here should return other errs as well.

[SpiritZhou]

Here should return other errs as well.

[BojanZelic]

@SpiritZhou I fixed the PR from your comments and updated the helmchart & documentation 🙏 please take a look whenever you get a chance.