add ability to apply trait to inline inputs/outputs

[lenguyenthanh]

I would love to have ability to apply trait to inline input/output for examples:

@http(method: "GET", uri: "/api/api", code: 200)
operation SomeOperator {

  @sometrait()
  input := {
    x: String
  }
}

[kstich]

This is supported, just in a slightly different location (see Smithy IDL spec on inline I/O):

@http(method: "GET", uri: "/api/api", code: 200)
operation SomeOperator {
    input := @sometrait( ... ) {
        x: String
    }
}