Infer allowed() values for parameters

[afscrome]

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

I often find myself implementing the following pattern in bicep - an input parameter and a map to convert it to another value. In this scenario the values in the @allowed() attribute and the properties on the map need to remain in sync.

@allowed([
  'Prod'
  'NonProd'
])
param environment string

var envShortNameMap = {
  Prod: 'pd'
  NonProd: 'np'
}

var envShortName = envShortNameMap[environment]

This pattern mostly works fine, until adding a new value - it's too easy to update either the parameter allow list or the map without updating hte other.

Describe the solution you'd like
It would be really nice if bicep could automatically infer the @allowed() values for the environment input string. This would reduce duplication in the code, ensuring the values remained in sync.

Fixing #5045 would partially help resolve some of the pain of keeping values in sync as it should at least result in a compiler error if I updated the @allowed() attribute but forgot to update envShortNameMap, but it wouldn't help if I updated envShortNameMap without updating the @allowed() attribute

[alex-frankel]

Related to #4158, but it would be interesting to leverage something similar keyof in TypeScript.

[shenglol]

Adding an example showing how keyof can be used:

param environment keyof evnShortNameMap // 'Prod' | 'NonProd'

var envShortNameMap = {
  Prod: 'pd'
  NonProd: 'np'
}

var envShortName = envShortNameMap[environment]

[brwilkinson]

Assuming this could also work with:

var envShortNameMap = json(loadTextContent('environmentInfo.json'))