OpenAPI v3.1 – The best thing to happen to OpenAPI since OpenAPI

## Changelog

The below changes are documented as part of the release notes.

### Additions

1. Introduced a new top-level field - `webhooks`. This allows describing out-of-band webhooks that are available as part of the API.

2. The Info Object has a new `summary` field.

3. The License Object now has a new `identifier` field for SPDX licenses.

4. Components Object now has a new entry pathItems, to allow for reusable Path Item Objects to be defined within a valid OpenAPI document.

5. Added the `jsonSchemaDialect` top-level field to allow the definition of a default $schema value for Schema Objects.

### Extended Functionality

1. Updated primitive types to be based on JSON Schema Specification Draft 2020-12. This now includes type null.

2. Lifted the restriction of allowing Request Body only in HTTP methods where the HTTP 1.1 specification RFC7231 has explicitly defined semantics for. While allowed in other methods, it is not recommended.

3. Added support to object type for spaceDelimited and pipeDelimited style values.

4. The Encoding Object now supports `style`, `explode` and `allowReserved` for multipart/form-data media type as well.

5. To enable better webhooks support, expressions in the Callback Object can now also reference Path Item Objects.

6. When using the Reference Object, `summary` and `description` fields can now be overridden.

7. The Discriminator Object can now be extended with Specification Extensions.

8. Added support for mutual TLS (mutualTLS) as a security scheme.

9. Used security requirements can now define an array of roles that are required for execution (and not only scopes for OAuth 2.0 security schemes).

### Changes

1. An OpenAPI Document now requires at least one of paths, components or webhooks to exist at the top level. While previous versions required paths, now a valid OpenAPI Document can describe only webhooks, or even only reusable components. Thus, an OpenAPI Document no longer necessarily describes an API.

2. Anywhere in the 3.0.0 Specification that had a type of Schema Object | Reference Object has been replaced to be Schema Object only. With the move to full JSON Schema support, $ref is inherently part of the Schema Object and has its own defined behavior.

3. Both `x-oai-` and `x-oas-` prefixes for Specification Extensions are now reserved to be defined by the OpenAPI Initiative.

4. `format` is now not validated by default.

### Breaking changes

1. The specification versioning no longer follows SemVer.

2. The nullable keyword has been removed from the Schema Object (null can be used as a type value).

3. `exclusiveMaximum` and `exclusiveMinimum` cannot accept boolean values (following JSON Schema).

4. Due to the compliance with JSON Schema, there is no longer interaction between `required` and `readOnly`/`writeOnly` in relation to requests and responses.

5. `format` (whether byte, binary, or base64) is no longer used to describe file payloads. As part of JSON Schema compliance, now contentEncoding and contentMediaType can be used for such specification.

6. Server Variable's `enum` now MUST not be empty (changed from SHOULD).

7. Server Variable's `default` now MUST exist in the enum values, if such values are defined (changed from SHOULD).

8. `responses` are no longer required to be defined under the Operation Object.

### Clarifications

1. Reworded the definition of OpenAPI Document to reflect that a document no longer must describe paths, but can describe either paths, webhooks, components or any combination of them.

2. Further updated Schema Object's description to take into account the latest draft, and the default use of https://spec.openapis.org/oas/3.1/dialect/base as the default OAS dialect.

3. Dropped the term RESTful APIs in favor of HTTP APIs.

4. Resolution of relative references has been redefined and clarified. Note there's a difference in resolution between Schema Object References and all others.

5. Modification of examples to improve them and provide context for new fields/objects.

6. Path parameter values cannot contain the unescaped characters /, ? or #.

7. Unified wording when values are URLs/URIs.

8. Reworded Path Item's $ref to take into account reference and component changes.

9. Minor text changes to improve consistency and readability.

10. Reworded "Schema Vocabularies" to "Schema dialects".

11. It is now clarified when path template expression may not have a corresponding path parameter.

12. A new section was added to address how to handle the `$schema` keyword (implicitly and explicitly).