Add the integrity field for dynamic plugin packages in the helm chart values schema

[Frank Kong]

What do you want to improve?

Add the newly added integrity field required for checking integrity of npm packages for dynamic plugins introduced in janus-idp/backstage-showcase#670.

"integrity": {
  "title": "Integrity checksum of the package. Optional for local packages. Supported algorithms include: `sha512`, `sha384` and `sha256`. Refer to https://w3c.github.io/webappsec-subresource-integrity/#integrity-metadata-description for more information",
  "type": "string"
}