`API().get_schema_type_mapping(args)` is missing information about a possible index signature in a schema

[diefans]

Bug Report

I want to programmatically inspect and deal with KCL schema information und stumbled upon the inability to know anything about the index signature of a schema.

1. Minimal reproduce step (Required)

create a file: test.k

schema Foo:

schema IndexSchema:
    [foo: str]: Foo

schema Test:
    attr: str
    index: IndexSchema

from google.protobuf.json_format import MessageToJson
from kcl_lib import api

exec_args = api.ExecProgram_Args(k_filename_list=["test.k"])
args = api.GetSchemaTypeMapping_Args(exec_args=exec_args)
result = api.API().get_schema_type_mapping(args)

print(MessageToJson(result))

2. What did you expect to see? (Required)

Some field indicating the name and type of the index signature [<index_attr_name>: <index_attr_type>]: <index_type> and also if it is open to contain yet unknown attributes using [...<index_attr_type>]: <index_type> where index_attr_name == "...".

3. What did you see instead (Required)

{
  "schemaTypeMapping": {
    "Foo": {
      "type": "schema",
      "schemaName": "Foo",
      "filename": "./test.k",
      "pkgPath": "__main__"
    },
    "IndexSchema": {
      "type": "schema",
      "schemaName": "IndexSchema",
      "properties": {
        "bar": {
          "type": "schema",
          "schemaName": "Foo",
          "line": 1,
          "filename": "./test.k",
          "pkgPath": "__main__"
        }
      },
      "required": [
        "bar"
      ],
      "filename": "./test.k",
      "pkgPath": "__main__"
    },
    "Test": {
      "type": "schema",
      "schemaName": "Test",
      "properties": {
        "index": {
          "type": "schema",
          "schemaName": "IndexSchema",
          "properties": {
            "bar": {
              "type": "schema",
              "schemaName": "Foo",
              "line": 1,
              "filename": "./test.k",
              "pkgPath": "__main__"
            }
          },
          "required": [
            "bar"
          ],
          "line": 2,
          "filename": "./test.k",
          "pkgPath": "__main__"
        },
        "attr": {
          "type": "str",
          "line": 1
        }
      },
      "required": [
        "attr",
        "index"
      ],
      "filename": "./test.k",
      "pkgPath": "__main__"
    }
  }
}

4. What is your KCL components version? (Required)

0.11.0-linux-amd64

[Peefy]

Related issue: kcl-lang/kcl-go#411 cc @He1pa @zong-zhe