Support returning plain values from Java methods

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Issue details

As a provider author I would like to be able to mark methods as plain: true so that my provider's consumers can access their results directly without an Output wrapper.

Further, methods that return a single value only should not be wrapped in a struct, making it as simple as possible to consume.

Methods should be able to return explicit Provider references usable for configuring other resources in the stack, so that methods can be used as factories to simplify complex provider configuration.

Here is a PR implementing plain-valued methods for Node, Python, Go that includes acceptance tests for this feature:
https://github.com/pulumi/pulumi/pull/13592

Borrowing from that PR, the new schema forms is listed below; "tlsProvider" returns a singleton provider reference, "meaningOfLife" returns a singleton integer, and "objectMix" returns both in a struct.

  "functions": {
    "metaprovider:index:Configurer/tlsProvider": {
      "inputs": {
        "properties": {
          "__self__": {
            "$ref": "#/resources/metaprovider:index:Configurer"
          }
        }
      },
      "outputs": {
        "$ref": "/tls/v4.10.0/schema.json#/provider",
        "plain": true
      }
    },
    "metaprovider:index:Configurer/meaningOfLife": {
      "inputs": {
        "properties": {
          "__self__": {
            "$ref": "#/resources/metaprovider:index:Configurer"
          }
        }
      },
      "outputs": {
        "type": "integer",
        "plain": true
      }
    },
    "metaprovider:index:Configurer/objectMix": {
      "inputs": {
        "properties": {
          "__self__": {
            "$ref": "#/resources/metaprovider:index:Configurer"
          }
        }
      },
      "outputs": {
        "type": "object",
        "plain": true,
        "properties": {
          "provider": {
            "$ref": "/tls/v4.10.0/schema.json#/provider"
          },
          "meaningOfLife": {
            "type": "integer"
          }
        }
      }
    }
  },

Affected area/feature

Start by reviewing PR 13592 and its acceptance tests for the plain-valued method behavior across Node, Python, and Go, then compare that design with the Java implementation. Done means Java methods support plain singleton values, provider references, and mixed object results using the described schema forms.