Reusable prop declaration

# Property declaration proposal

## Problem

We have found in Azure that there is some common properties that are to be used in a pick and choose manner. For example I want to include a retryAfter header or a eTag.

The current approach is to create this model that is then spread

```tsp
model ETagHeader {
@header eTag: string;
}
```

Then to use it with spread.

```tsp
op test(...ETagHeader): void;
```

The problem with this approach is that you cannot customize that property. You cannot use it with versioning. This means as soon as you want to add a decorator, documentation or anything else you then have to redefine the property.

## Proposal

Introduce a new syntax to declare reusable properties. Using the keyword `prop`(or `property` both are reserved right now) and then the same syntax as a normal property.

```tsp
@header
prop eTag: string;
```

Those properties then could be used just like normal properties.

```tsp
model MyModel {
eTag,
}
```

or given a new name

```tsp

model MyModel {
myETag: eTag,
}
```

## Examples

```tsp
model MyModel {
/** With some docs */
eTag,

retryAfterX: retryAfter,
}
```