[http-client-csharp] Deserialize additional information from TCGC output

A specific example is
```
model DurationModel {
@encode("duration-constant")
input: duration;
}
```
TCGC outputs:
```
"kind": "duration",
"name": "duration",
"encode": "duration-constant",
```
When we do `TypeSpecInputDurationTypeConverter`, we need extra logic to map or store this information to the downstream plugin, which we don't have it yet.

We currently only have `Enum.TryParse(encode, ignoreCase: true, out var encodeKind)`.