Add global key to values schema

A user reported they were having schema validation issues when using Big Bang's Keycloak chart as a dependency chart:

Error: values don't meet the specifications of the schema(s) in the following chart(s):
icam:
- at '': additional properties 'global' not allowed

This is due to Helm's internal behavior of adding a "global" object when a chart is used a dependency - https://github.com/helm/helm/issues/10392#issuecomment-1513258871.

While uncommon we should add an unstructured "global" object to the top level supported keys for this schema:

      "additionalProperties": false,
      "properties": {
       "global": {
            "type": "object"
        },