Essentials-openapi

- Fixes [22](https://github.com/Neoteroi/essentials-openapi/issues/22)

- Fixes [18](https://github.com/Neoteroi/essentials-openapi/issues/18)
- Workflow maintenance

- Changes how `httpx` version is pinned (`<1`)

- Adds support for OpenAPI specification files split into multiple files
https://github.com/Neoteroi/mkdocs-plugins/issues/5
- Adds support for `externalDocs` and `tags` root properties

- Adds a new output style, to provide an overview of the API endpoints with
PlantUML
- Fixes two bugs caused by improper handling of OpenAPI Documentation without
`components` https://github.com/Neoteroi/mkdocs-plugins/issues/9

- Adds features and a CLI to generate artifacts from OpenAPI Documentation
files (markdown for MkDocs and PyMdown extensions, PlantUML class diagrams
from components schemas)
- Drops support for Python 3.6