Make addons values schema more restrictive

The values schema has a very basic safeguard for ensuring that only "known" keys can be specified for the top level values.

This prevents you from being able to do something like:

$ helm template chart --set=foo=bar       
Error: values don't meet the specifications of the schema(s) in the following chart(s):
bigbang:
- at '': additional properties 'foo' not allowed

This works because we set additionalProperties: false at the top level: https://repo1.dso.mil/big-bang/bigbang/-/blob/3.8.0/chart/values.schema.json?ref_type=tags#L40.

A very easy additional safeguard would be adding additionalProperties: false to the addons object. This should be possible because all addons keys are known.

This would cause this to fail schema validation (this currently passes):

helm template chart --set=addons.foo=bar