Can `x-ms-azure-resource ` be safely removed

[timotheeguerin]

There is some spec still using it and seems like csharp autorest generator might make use of it.

1. Figure out if this is actually useful
2. If it is, why are spec using it not using the base resource types
3. If they have valid legacy reason then we need some deprecated decorator to get parity

[markcowl]

• This is still used by older emitters for c-sharp
• We need an ARM decorator that will be consumed by autorest to add this extension
• The decorator should be called @customAzureResource and have an associated linting rule that discourages its usage (this can be excluded from legacy rulesets)