Essentials-openapi

- Restores support for enums on examples `dataclasses`, after the fix
implemented in `0.1.3`
- Adds support for built-in `UUID`, `time`, `date`, `datetime`, `bytes`,
handling in examples for `YAML` format
- Adds `partial-time` ValueFormat for `time` (see
https://xml2rfc.tools.ietf.org/public/rfc/html/rfc3339.html#anchor14)

- Corrects a bug forcing `camelCase` on examples objects handled as dataclasses
- Adds base64 ValueFormat to the v3 enum

- Adds a changelog
- Adds a code of conduct
- Updates `PyYAML` dependency to version `5.4.1`