Add an HTTP endpoint for DApp to check connection availability

[0xtoorich]

According to our statistics, approximately 20% of our user base is unable to connect to the official TON Connect bridge. Therefore, we need an HTTP endpoint to help DApp developers (Telegram mini apps) quickly check whether they can successfully establish a network connection.

We recorded 34,000 connection attempts within one hour, of which 7,600 were unsuccessful. Therefore, providing a connectivity check endpoint seems quite urgent. I hope this can be added. Thank you!