Add Steam ticket auth for auth=steam HTTP requests

[Ash-bass]

Adds support for Steam-authenticated HTTP requests. When a URL includes auth=steam, the client sends the user's Steam session ticket in the Authorization header so the server can verify the request. This works for downloads, API calls, and any other HTTP request that goes through the shared web client (e.g. content downloads, server content, metadata fetching).

Changes:

a4f081c:

• AcManager.Tools/Helpers/SteamTicketProvider.cs (new) obtains Steam session ticket via GetAuthSessionTicket and exposes UrlRequiresSteamTicket() to detect auth=steam in URLs.
• AcManager.Tools/Helpers/CookieAwareWebClient.cs adds Steam ticket injection in GetWebRequest, setting Authorization: Bearer <hex-ticket> when the URL contains auth=steam.
• AcManager.Tools/AcManager.Tools.csproj adds compile include for SteamTicketProvider.cs.

[Ash-bass]

The main purpuse of this is locking content download behind and auth, so with the steam ticket, server can verify its legitimacy, and then provide specific content to an user or group of users based on steam id, but ye can be used for multiple things.

P.S: This is my first pull request — please excuse any inconvenience. Feedback is welcome.