feat: expose conversation handle to plugins

[KnorpelSenf]

The plugins option allows plugins to be installed inside a conversation, and external allows the conversation to read the session data. It was pointed out in https://t.me/grammyjs/291998 that it is currently impossible to access the session data from inside a plugin.

This PQ introduces a plugins option that has access to the conversation handle. That way, the following code becomes possible.

bot.use(createConversation(convo, {
  plugins: conversation => [async (ctx, next) => {
    // can now install session data on the context:
    ctx.sess = await conversation.external(ctx => ctx.session)
  }]
}))