Add API method-specific extension functions

[anfanik]

This pull request adds method-specific functions. (originally from sktb-spring-boot-starter)
It's my vision of a Kotlin-way API usage.

It based on #409 and #408 and there are only two diffs: 5a994fa, 25ade71

It looks similar to #331, but without breaking changes and moving client-specific logic to request classes

Added:

• TelegramAware interface
  All extensions are made for this interface.

  Use case:
  sktb-spring-boot-starter has a TelegramBot wrapper – TelegramService.kt

  This interface allows to implement it and use all extensions even in wrapper.

• Extension for API methods (what I use most often in my projects, todo: add extensions for every method in furture releases)

  Example:

  You can use:

  telegram.sendMessage(chatId = "anfanik", text = "Hello world!") {
      parseMode(ParseMode.HTML)
  }

  or general execute function for not-implemented API methods:

  telegram.execute(SendMessage("anfanik", "Hello world!")) {
      parseMode(ParseMode.HTML)
  }

  instead of more complex or Java-style usages:

  telegram.execute(SendMessage("anfanik", "Hello world!").apply {
      parseMode(ParseMode.HTML)
  })
  
  telegram.execute(SendMessage("anfanik", "Hello world!").parseMode(ParseMode.HTML))

  With the Request Property Delegates (Bot API v8.0 #408), it will allow to build requests by setting properties, for example:

  telegram.sendMessage(chatId = "anfanik", text = "Hello world!") {
      parseMode = ParseMode.HTML
      replyMarkup = InlineKeyboardMarkup()
      ...
  }

• RequestPreprocessor interface
  A functional interface. It should be called before any request execution.
  By default, it does nothing, but it can be used to modify request before execution.

  Use case:
  sktb-spring-boot-starter has a FormatUtility with default formatting options – FormatUtility.kt

  This formatting settings are applied in extension functions by default in TelegramAwareExtension.kt

  RequestPreprocessor makes possible to apply sktb-default formatting options and still use buildin extensions

[pengrad]

Looks great!
Seems enough to merge only this one as it contains both v8 and v8.1