## Summary

Adds `discriminatedUnion` method to `@kbn/config-schema` for
discriminated union of object schemas.

```ts
const exampleType = schema.discriminatedUnion('type', [
  schema.object({ type: schema.literal('str'), string: schema.string() }),
  schema.object({ type: schema.literal('num'), number: schema.number() }),
  schema.object({ type: schema.literal('bool'), boolean: schema.boolean() }),
]);
```

Order of Errors:
1) missing `discriminator` property.
1) `discriminator` property has wrong type (only strings via
`schema.literal` and one `schema.string` are permitted)
1) `discriminator` is a string but not one of the expected
discriminators.
1) validation errors from the respective schema based on
`discriminator`, only the matching schema is validated.


Also allows for fallback schema. Limit of one fallback, position in
array does not matter.

```ts
const exampleType = schema.discriminatedUnion('type', [
  schema.object({ type: schema.literal('str'), string: schema.string() }),
  schema.object({ type: schema.literal('num'), number: schema.number() }),
  schema.object({ type: schema.string }, { unknowns: 'allow' }), // allows any other type other than 'str' and 'num'
]);
```

## Error Example

```
{ type: 'str', string: 123 } // "string" property is a number instead of a string

[string]: expected value of type [string] but got [number]
```

closes #84264

### Checklist

- [x]
[Documentation](https://www.elastic.co/guide/en/kibana/master/development-documentation.html)
was added for features that require explanation or tutorials
- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios
- [x] This was checked for breaking HTTP API changes, and any breaking
changes have been approved by the breaking-change committee. The
`release_note:breaking` label should be applied in these situations.
- [x] The PR description includes the appropriate Release Notes section,
and the correct `release_note:*` label is applied per the
[guidelines](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)
- [x] Review the [backport
guidelines](https://docs.google.com/document/d/1VyN5k91e5OVumlc0Gb9RPa3h1ewuPE705nRtioPiTvY/edit?usp=sharing)
and apply applicable `backport:*` labels.

## Summary

Close #181994

Support generating OAS for `@kbn/config-schema`'s using our new
`schema.discriminatedUnion` type
#246095

## Notes

* Force developers to define a `{ meta: { id: '...' } }` for the objects
inside a `schema.discriminatedUnion` so that we can generate the correct
OAS. Guidance for name is as follows: `<your-id>-<your-team-or-area>`.
The intention is that IDs are globally unique while remaining somewhat
user readable as they will be surfaced in our docs.
* Tackled a few unrelated OAS/schema chores in this PR

## Conversion

Given a schema like:

```ts
    schema.discriminatedUnion('type', [
      schema.object(
        { type: schema.literal('str'), value: schema.string() },
        { meta: { id: 'my-str-my-team' } }
      ),
      schema.object(
        { type: schema.literal('num'), value: schema.number() },
        { meta: { id: 'my-num-team' } }
      ),
    ]),
```

Produce OAS like:

```js
    {
      oneOf: [
        {
          $ref: '#/components/schemas/my-str-my-team',
        },
        {
          $ref: '#/components/schemas/my-num-team',
        },
      ],
      discriminator: {
        propertyName: 'type',
      }
    }
...
components: {
  schemas: {
      'my-str-my-team': {
        type: 'object',
        properties: {
          type: { type: 'string', enum: ['str'] },
        },
      },
      'my-num-team': {
        type: 'object',
        properties: {
          type: { type: 'string', enum: ['num'] },
        },
      },
    },
}
```