Add VCR for specs

[AlexWayfer]

More info here: https://benoittgt.github.io/vcr/

I had some experience with it, so I could set it up after merging my existing pull requests.

Main benefits:

• no more BOT_API_TOKEN is required for specs (currently we even don't have instructions for it in README);
• we're not testing third-party server (Telegram) availability (we'll able to run specs even in outages);
• speed of specs, obviously.

Cons:

• new files in git repository (with masked secrets, like tokens);
• we have to watch for Telegram API updates manually and update our cassettes (or have some TTL for them, but regularly it's not required).

[AlexWayfer]

Meh, for now I'm unable to do this, because login into test environment is not working: https://github.com/telegramdesktop/tdesktop/issues/26361

I've written to @BotSupport, but don't expect a lot.

If somebody can provide me a key for bot in the test environment — I'd appreciate this.