feat(http-client-csharp): encode int as string

[archerzz]

Clear and concise description of the problem

This is part of Azure/autorest.csharp#4919

As defined in Azure/typespec-azure#1179, we need to support @encode an int into string, like below:

model Foo {
  @encode(string)
  x: int
}

Checklist

• Follow our Code of Conduct
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.