Make it possible to use latest type declarations without upgrading bicep

[blackchoey]

Is your feature request related to a problem? Please describe.

I'm using bicep to register outlook channel for bot service (a feature from #5085). Since I'm using an old bicep version, bicep build failed because it thinks OutlookChannel is an invalid value. I need to use the latest bicep release to build the bicep file I authored. Here's the bicep snippet I wrote:

resource botServiceOutlookChannel 'Microsoft.BotService/botServices/channels@2021-05-01-preview' = {
  parent: botService
  location: 'global'
  name: 'OutlookChannel'
  properties: {
    channelName: 'OutlookChannel'
  }
}

Describe the solution you'd like
I would expect bicep can check whether there's any type updates and download the latest type automatically when executing build command. So I can reduce the frequency of updating bicep binary.

In the meantime, I'm thinking whether it's possible to read the declaration from internet. So people does not need to wait for a new bicep type declaration after Azure service released a new feature. However this may introduce some performance issue.

[alex-frankel]

Dup of #5453