feat(nextcloud): Add support for AWS session token key

[refucktor]

Description of the change

• Introduced a new configuration option for AWS Credentials to use a Session Token.
• Updated template and values to enable usage of the new key

Benefits

• Support using temporal AWS Credentials (e.g. STS assumed role)

Possible drawbacks

• None

Applicable issues

• resolves #690
• related:
  • server #50702
  • docker #2364

Additional information

• As you might notice, the session token is only referenced within an existing secret config, the nature of such session tokens comes from temporal credentials, more likely secrets auto-generated from sts assume role, etc. Therefore it only makes sense to reference it from an "existing secret"

Checklist

• [x] I have read the CONTRIBUTING.md doc.
• [x] DCO has been signed off on the commit.
• [x] Chart version bumped in Chart.yaml according to semver.
• [x] (optional) Parameters are documented in the README.md