sites/siteExtensions properties incorrectly reported as read-only

**Bicep version**
0.4.613

**Describe the bug**
'Microsoft.Web/sites/siteExtensions@2021-01-15' lists properties as read-only. This is not the case in ARM, where the version property can be used to specify the version of the extenstion to install on the app.

**To Reproduce**
```bicep
resource siteExtension 'Microsoft.Web/sites/siteExtensions@2021-01-15' = [for extension in site.extensions: {
name: extension.name
parent: siteRes
properties: {
version: extension.version
}
}]
```

Start with the Bicep reproducer for Microsoft.Web/sites/siteExtensions@2021-01-15 and inspect how its properties are classified. Confirm that the version property can be assigned as shown while other read-only reporting remains correct, then add or update coverage for this resource behavior.

Written by the indexing model from the issue text.