Forbid nonsensical flag interactions in the Pulumi schema

[iwahbe]

Hello!

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Issue details

We have several type flags that can be set on a single type in the schema. These include:

• const: Has a constant and specific value, similar to a const variable. => The caller should not specify a value.
• default: The field has a default value if no value is provided. => The caller may specify a value.
• required: The user must specify a value at the call site. => The caller must specify a value.

In the past, we allowed the Pulumi schema to specify nonsensical combinations of flags such as

"field_name": {
          "type": "string",
          "const": "val",
          "default": "another",
	  "required": true
        }

To ensure that the schema is coherent, and that we can faithfully respect the schema during codegen, we will disable specifying any combination of const, default, and required (just 1 is ok).

Note: The required property is not set on the property itself but in an associated list.

Affected area/feature

This will narrow the set of viable schema inputs. It may allow us to simplify codegen, because it strengthens what assumptions we can make.

[iwahbe]

This will require a change in schema generation for pulumi-azure-native at the very least, as it generates types like this:

        "azure-native:datafactory:AzureDatabricksDeltaLakeSink": {
            "description": "A copy activity Azure Databricks Delta Lake sink.",
            "properties": {
                ...
                "type": {
                    "type": "string",
                    "description": "Copy sink type.\nExpected value is 'AzureDatabricksDeltaLakeSink'.",
                    "const": "AzureDatabricksDeltaLakeSink"
                }
            ...
            },
            "type": "object",
            "required": [
                "type"
            ]
        }