Add optional configuration parameter to select JSON schema version

[hpmellema]

Issue #, if available: issue #1252

Description of changes:
Adds an optional configuration parameter jsonSchemaVersion to select the version of JSON schema to use. The JSON schema version will still default to draft07, but can be set to draft2020-12. This new configuration parameter will also provide a mechanism to support future JSON schema changes.

Reviewing the JSON schema specifications, it appears that the only change needed was to update "definitions" -> "$defs" as Smithy jsonSchema does not support meta-schema generation, the use of plain-name fragments, tuples, or dynamic anchors.

Cloudformation
Cloudformation only supports jsonSchema draft07 at this time (see here)
In order to prevent compatibility issues, a check was added to the cloudformation plugin to ensure only JSON Schema draft07 is used in the configuration. I expect we will update this as Cloudformation and API Gateway add support for later versions.

OpenAPI
The OpenAPI plugin was updated to set the JSON Schema version parameter based on the latest version supported by the OpenAPI spec version.
I.e. OpenAPI3.1 will use draft2020-12 while OpenAPI3.0 will use draft07

Testing

• manually tested that both OpenAPI and Cloudformation plugins correctly used the new configuration parameter and rejected unsupported versions/parameters
• Added a new integration test for draft2020-12 in the JSON schema module
• Added test that OpenAPI config correctly associates with the right JSON schema version

JSON Schema Change list
The following two web pages detail the
Schema Changes:

• draft07 -> draft2019-09
• draft2019-09 -> draft2020-12

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.