Improve validation and completions with additionalProperties

[majastrz]

Is your feature request related to a problem? Please describe.

• We should offer property access completions for the "additional" properties that have actually been specified in the resource declaration.
• Property access on an "additional" property that isn't specified in the resource declaration should produce a warning. (Can't be an error because RPs can violate the RPC and return a modified set of properties.)

resource bp 'Microsoft.Blueprint/blueprintAssignments@2018-11-01-preview' = {
  name: 't'
  location: resourceGroup().location
  identity: {
    type: 'None'
  }
  properties: {
    parameters: {
      'foo': {
        reference: {
          keyVault: {
            id: 's'
          }
          secretName: 's'
        }
      }
    }
    resourceGroups: {
      'bar': {
        name: 's'
        location: 's'
      }
    }
  }
}

// no name completions
var test = bp.properties.resourceGroups.|

// prop. access completions work at `|` but no warning on "blah"
var test2 = bp.properties.resourceGroups.blah.

This originally was discovered during #3084