Feature: split definitions to several swagger.json

**Describe the issue**
We have several azure functions that are used for different applications. In order to offer a reduced definition to each consumer, we would like to be able to group the functions and generate a separate document per group. There are already the tags and it would be cool to be able to enable that one document is generated per tag.

Alternatively, we'd like to have a separate property that can be used explicitly for this and allow a function to be transferred to multiple documents.

**Expected behavior**
- There is a possibility to activate multiple documents (swagger.json).
- There is a property that determines which function is included in which document.
- A function can be included in 1 or more documents.
- Startingpoint: https://github.com/domaindrivendev/Swashbuckle.AspNetCore#generate-multiple-swagger-documents

The issue names no repository files or tests. Start with the linked Swashbuckle.AspNetCore multiple-document example, then trace the extension's OpenAPI generation entry point. Define document activation and function-to-document mapping, including multiple documents per function, and verify that each generated swagger.json contains the expected functions.

Written by the indexing model from the issue text.