New Feature: Expose UI endpoints to APIM by devs' choice

Curerntly, 4 UI-related endpoints are not exposed to the OpenAPI document:

* `/swagger/ui`
* `/swagger.{extension}`
* `/openapi/{version}.{extension}`
* `/oauth2-redirect.html`

There are cases that devs want to expose those endpoints through Azure APIM. In that case, they have to manually add them. To reduce this manual efforts, this feature would be useful:

1. Add the environment variable `OpenApi__ExposeUI` – its value will be `false` by default. If it's set to `true`, all four UI-related endpoints should be accessible through APIM.
2. Add the querystring parameter `exposeui` – itsvalue will be `false` by default. If it's set to `true`, all four UI-related endpoints should be accessible through APIM. In addition to that, this querystring parameter should take precedense to the environment variable.

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Update: 2022-03-29

There have already been options – `OpenApi__HideSwaggerUI` and `OpenApi__HideDocument` that are applicable on the function app levels. To avoid providing dev with misleading information, the environment variable names might be changed.

Start by locating the routing for `/swagger/ui`, `/swagger.{extension}`, `/openapi/{version}.{extension}`, and `/oauth2-redirect.html`, then inspect how `OpenApi__HideSwaggerUI` and `OpenApi__HideDocument` are applied. Define the precedence between the `exposeui` query parameter and `OpenApi__ExposeUI`, and verify that all four endpoints are exposed only when the effective value is true.

Written by the indexing model from the issue text.