Add nested objects support

[Sergej-Popov]

Adds support for nested objects, this allows to create a schema for arbitrary dictionary objects where the nested structure is defined but keys are not. e.g OpenAPI spec

  "paths": {
    "/my-resource": {
      "post": {
        "tags": [
          "resource"
        ],
        "summary": "Create my Resource"

In the above example, resource structure is well defined, but the path identifiers are not.

The second commit in this PR adds support for double nested dictionaries. e.g:

{
      NestedDictionaryProp: {
        "KeyA": {
          "KeyA1": {
            DictionaryChildNumberProp: 500
          }
        }
      }
    }

Type of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• I have updated the documentation accordingly.

New Feature Submissions:

• Does your submission pass tests?
• Have you lint your code locally prior to submission?

Changes to Core Features:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your core changes, as applicable?
• Have you successfully ran tests with your changes locally?

[bingnz]

Nice! One suggestion I have is that perhaps you could wrap the schema objects using more fluent static 'factories'. For example,

const schema = new Schema<Model>()
        .with(m => m.StringProp, new AnyOf([
          "specificValueNotMatchingRegex",
          /^[A-z]+\.[A-z]+$/,
          new StringSchema({
            enum: ["123"]
          })
        ]))
        .build();

could be expressed as something like

      const schema = new Schema<Model>()
        .with(m => m.StringProp, jsonschema.matching.anyOf([
          "specificValueNotMatchingRegex",
          /^[A-z]+\.[A-z]+$/,
          jsonschema.string.exactMatch("123")
        ]))
        .build();

[Sergej-Popov]

I like static factories idea. I will create an issue to track that separately.