define encoding StringIndex model

[chunyu3]

Some Azure services return substring offset & length values within a string. For example, the offset & length within a string to a name, email address, or phone number.
https://github.com/microsoft/api-guidelines/blob/vNext/azure/ConsiderationsForServiceDesign.md#returning-string-offsets--lengths-substrings
When the string is encoding in different format (UTF-8, UTF-16 or CodePoint ), the offset/length will be different.

In order make sure client SDK of different language can correct parse the string, service will return a model to indicate the offset/length for different encoding. e.g.

@doc("String index encoding model.")
model StringIndex {
  @doc("The offset or length of the substring in UTF-8 encoding")
  utf8: int32;

  @doc("""
    The offset or length of the substring in UTF-16 encoding.
    
    Primary encoding used by .NET, Java, and JavaScript.
    """)
  utf16: int32;

  @doc("""
    The offset or length of the substring in CodePoint encoding.
    
    Primary encoding used by Python.
    """)
  codePoint: int32;
}

https://github.com/microsoft/api-guidelines/blob/685e493d38f8a3875c22336dcd177f4b54dcfb23/azure/Guidelines.md#returning-string-offsets--lengths-substrings

There is a feature request Azure/autorest.csharp#4925 to wrap encoding stringIndex model to int, e.g.

// Currently
taggerOutput.Offset.Utf16 // int
taggerOutput.Offset //  {"utf8": 10, "utf16":10, "codePoint:10"} - StringIndex model

// Desired
taggerOutput.Offset // int -- While still supporting the serialization of the StringIndex model

To implement this feature, codegen need to recognize the model is StringIndex model.

• option 1 : Typespec define a decorator to indicate that this is for offset/length indexing model for different encoding
• option 2: TypeSpec define a common model for this