Change OpenAPI document name resolution to be case-insensitive

[sander1095]

Change OpenAPI document name resolution to be case-insensitive

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Case-sensitive urls are now supported when retrieving OpenAPI documents.

Description

There was a routing issue when using case-sensitive document names in Microsoft.AspNetCore.OpenApi's AddOpenApi() and MapOpenApi() methods. If you registered a document in a case-sensitive manner (MyDocument) and tried to retrieve it at /openapi/mydocument.json, it would result in a 404.

This is odd, as ASP.NET Core's routing has always been case-insensitive by default.

The reason for this bug is that keyed services are used to register relevant services. The key is the document name, and keyed services are case-sensitive.

This PR changes this inconsistent behaviour by allowing case-insensitive document resolution by invariantly lowercasing both the registered document name and the document name that's being retrieved.

Fixes #59175