Reduce the use of oneOf in the schemas where possible

[massfords]

Using oneOf can result in a large number of errors from the validator. I'd like to replace a few of these where the same validation could be done easily with a TS function.

Consider the requirement of Next being mutually exclusive with End

This is achieved in the Task with a few lines in the schema:

{
  "oneOf": [{
    "required": ["Next"]
  }, {
    "required": ["End"]
  }],
}

Here's the error message from the test definition valid-task-timer.json after altering the Task to have both End and a valid Next value.

Spoiler..... It's a long list of errors

[
  {
    "instancePath": "/States/Send SNS Message",
    "schemaPath": "#/required",
    "keyword": "required",
    "params": {
      "missingProperty": "Choices"
    },
    "message": "must have required property 'Choices'"
  },
  {
    "instancePath": "/States/Send SNS Message",
    "schemaPath": "fail.json#/additionalProperties",
    "keyword": "additionalProperties",
    "params": {
      "additionalProperty": "Resource"
    },
    "message": "must NOT have additional properties"
  },
  {
    "instancePath": "/States/Send SNS Message",
    "schemaPath": "#/oneOf",
    "keyword": "oneOf",
    "params": {
      "passingSchemas": [
        0,
        1
      ]
    },
    "message": "must match exactly one schema in oneOf"
  },
  {
    "instancePath": "/States/Send SNS Message",
    "schemaPath": "#/oneOf",
    "keyword": "oneOf",
    "params": {
      "passingSchemas": [
        0,
        1
      ]
    },
    "message": "must match exactly one schema in oneOf"
  },
  {
    "instancePath": "/States/Send SNS Message",
    "schemaPath": "#/additionalProperties",
    "keyword": "additionalProperties",
    "params": {
      "additionalProperty": "Resource"
    },
    "message": "must NOT have additional properties"
  },
  {
    "instancePath": "/States/Send SNS Message",
    "schemaPath": "#/oneOf",
    "keyword": "oneOf",
    "params": {
      "passingSchemas": [
        0,
        1
      ]
    },
    "message": "must match exactly one schema in oneOf"
  },
  {
    "instancePath": "/States/Send SNS Message",
    "schemaPath": "#/oneOf",
    "keyword": "oneOf",
    "params": {
      "passingSchemas": [
        0,
        1
      ]
    },
    "message": "must match exactly one schema in oneOf"
  },
  {
    "instancePath": "/States/Send SNS Message",
    "schemaPath": "#/oneOf/0/required",
    "keyword": "required",
    "params": {
      "missingProperty": "ItemProcessor"
    },
    "message": "must have required property 'ItemProcessor'"
  },
  {
    "instancePath": "/States/Send SNS Message",
    "schemaPath": "#/oneOf/1/required",
    "keyword": "required",
    "params": {
      "missingProperty": "Iterator"
    },
    "message": "must have required property 'Iterator'"
  },
  {
    "instancePath": "/States/Send SNS Message",
    "schemaPath": "#/oneOf/2/required",
    "keyword": "required",
    "params": {
      "missingProperty": "Iterator"
    },
    "message": "must have required property 'Iterator'"
  },
  {
    "instancePath": "/States/Send SNS Message",
    "schemaPath": "#/oneOf/3/required",
    "keyword": "required",
    "params": {
      "missingProperty": "ItemProcessor"
    },
    "message": "must have required property 'ItemProcessor'"
  },
  {
    "instancePath": "/States/Send SNS Message",
    "schemaPath": "#/oneOf",
    "keyword": "oneOf",
    "params": {
      "passingSchemas": null
    },
    "message": "must match exactly one schema in oneOf"
  },
  {
    "instancePath": "/States/Send SNS Message",
    "schemaPath": "#/oneOf",
    "keyword": "oneOf",
    "params": {
      "passingSchemas": null
    },
    "message": "must match exactly one schema in oneOf"
  }
]

The check provided by the schema could be done with a visit of the graph in the same way some of the other checks are implemented. This would provide an opportunity to report a simple message that a State cannot have both Next and End

I propose starting with this simple oneOf and replace it with a TS checker.