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

[pshao25]

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<DurationKnownEncoding>(encode, ignoreCase: true, out var encodeKind).

[live1206]

As offline discussed, @encode("duration-constant") is Azure specific, which should not be handled in MTG.
And the input types and corresponding serialization is not extensible for sub-plugins, the extensible way to deal with this is via Decorators.

typespec/packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.Input/src/InputTypes/DurationKnownEncoding.cs

Line 8 in bfbc8d4

Iso8601, Seconds, Constant

Constant is not accurate for the TypeSpec encoding, and it is only used for Azure.
We need to find a way(such as DurationKnownEncoding.Unknown) to map it instead of throwing
And then, Azure/MPG plugin can deal with decorators after deserialization.

cc @JoshLove-msft

[live1206]

Another proposal from @ArcturusZhang is to change the encode from enum to string, similar to

typespec/packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.Input/src/InputTypes/InputPrimitiveType.cs

Lines 30 to 31 in 839a03e

	public static InputPrimitiveType Base64 { get; } = new(InputPrimitiveTypeKind.Bytes, "bytes", "TypeSpec.bytes", BytesKnownEncoding.Base64);
	public static InputPrimitiveType Base64Url { get; } = new(InputPrimitiveTypeKind.Bytes, "bytes", "TypeSpec.bytes", BytesKnownEncoding.Base64Url);

Then, we should update TCGC duration-related models accordingly to be aligned.

[ArcturusZhang]

We could change it to an extensible enum so that it can accept new values.
And as a follow up, we need to consolidate #4228 as well and make sure we could extend the serialization logic

[JoshLove-msft]

We should add details as to where the fix needs to happen.