Allow @schema/nullable alongside @schema/type any=True

[pivotaljohn]

(From #707 synthesis: "Allow both nullable and type any=True")

---

In #400 (implemented in #517), we specifically disallowed both @schema/type and @schema/nullable.

We have now found a use-case where we'd like to allow it:

#@data/values-schema
---
#! Define which storage backend is used for registry and chartmuseum to store
#! images and charts.
#@schema/validation one_not_null=["filesystem", "azure", "gcs", "s3"]
imageChartStorage:
    #@schema/nullable
    filesystem:
      rootdirectory: /storage
      #! maxthreads: 100
    #@schema/nullable   
    azure:
      accountname: "" 
      accountkey: "" 
      container: "" 
      realm: "" 
    #@schema/nullable
    gcs:
      bucket: "" 
      encodedkey: "" 
      rootdirectory: "" 
    #@schema/type any=True
    #@schema/nullable
    s3:
      region: "" 
      bucket: "" 
      accesskey: "" 
      secretkey: "" 

Fails with:

ytt: Error: Overlaying data values schema (in following order: schema.yaml):
Invalid schema
==============

@schema/nullable, and @schema/type any=True are mutually exclusive
schema.yaml:
     |
  22 |     #@schema/type any=True
  23 |     #@schema/nullable
  24 |     s3:
     |

     = found: both @schema/nullable, and @schema/type any=True annotations
     = expected: one of schema/nullable, or schema/type any=True

However — as became evident in our usability testing (i.e. #707) — it is very natural to want to both set the default value for a map item to be null and want to allow any type.