Add Webhook Functionality

[virtualstaticvoid]

This PR adds the capability to receive Telegram updates via Webhook delivery.

Following the pattern established in the Updater class, the Webhook class follows similar steps for setup and running.

# define handlers as before
start <- function(bot, update) {
 bot$sendMessage(
   chat_id = update$message$chat_id,
   text = sprintf(
     "Hello %s!",
     update$message$from$first_name
   )
 )
}

# instantiate Webhook
webhook <- Webhook("https://example.com/webhook", Sys.getenv("TOKEN"), verbose = TRUE)

# wire up handler
webhook <- webhook + CommandHandler("start", start)

# start polling
webhook$start_server()

# OR, start polling, binding to all interfaces on port 3000
webhook$start_server(host = "0.0.0.0", port = 3000)

# stop polling
webhook$stop_server()

A working example application can be found at virtualstaticvoid/heroku-telegram-bot-webhook which is deployed to Heroku.

Start a chat with HerokuRBot to try it out. Send /plot command to receive a simple pie chart rendered in R.

All Submissions:

• [x] Have you followed the guidelines in our Contributing document?
• [x] Have you checked to ensure there aren't other open Pull Requests for the same update/change?

New Feature Submissions:

1. [x] Does your submission pass tests?
2. [x] Have you lint your code locally prior to submission?
3. [x] Documentation has been generated?

Changes to Core Features:

• [x] Have you added an explanation of what your changes do and why you'd like us to include them?
• [x] Have you written new tests for your core changes, as applicable?
• [x] Have you successfully ran tests with your changes locally?

Proposed Changes

• Implements Webhook capabilities as per Marvin's Marvellous Guide to All Things Webhook.
• Extends the Bot$set_workhook method to include all supported arguments; including ip_address, drop_pending_updates and secret_token.
• Fixes Bot.request method so that when an error occurs, it is reported with the response text, which helps when debugging.
• Bumps the package version to v2.4.3

[virtualstaticvoid]

I realised I haven't updated the main README page to include details of the new Webhook class, so I will add that shortly.