Created Secret token validator middleware

[BlueXTX]

resolve issue #1112

[karb0f0s]

First of all, @BlueXTX thank you for your contribution!
Though I have a few concerns about this change:

• The biggest issue is ASP.NET dependencies. I'm not sure how they play out for legacy platforms, covered by .netstandard2.0 target. It might be safer to introduce this change under #if DOTNET6_OR_GREATER directive to avoid collision with older .NET frameworks.
• I don't see a way to differentiate between controllers for this middleware. If I have other endpoints in my app, they would require X-Telegram-Bot-Api-Secret-Token header too.
• IMO helper functions and middleware classes would greatly benefit from some level of XML documentation on them.

@tuscen WDYT?

[BlueXTX]

First of all, @BlueXTX thank you for your contribution! Though I have a few concerns about this change:

• The biggest issue is ASP.NET dependencies. I'm not sure how they play out for legacy platforms, covered by .netstandard2.0 target. It might be safer to introduce this change under #if DOTNET6_OR_GREATER directive to avoid collision with older .NET frameworks.
• I don't see a way to differentiate between controllers for this middleware. If I have other endpoints in my app, they would require X-Telegram-Bot-Api-Secret-Token header too.
• IMO helper functions and middleware classes would greatly benefit from some level of XML documentation on them.

@tuscen WDYT?

1. Microsoft.Extensions.Options and Microsoft.AspNetCore.Http.Abstractions have support for netstandard2.0.
2. Will it be enough if I add configuration like this? app.UseWhen(context => context.Request.Path.StartsWithSegments("/YourPath"), appBuilder => { appBuilder.UseMiddleware<SecretTokenValidatorMiddleware>(); });

[tuscen]

I don't think we should take dependency on M.E.x libraries in the core package. It's ok to make a separate package for ASP.NET Core integration though.