Add `filename` parameter to MultipartBody

Overview

Add support for MultipartBody filename parameter

Related Issue

Fixes #398

Demo

N/A

Notes

One possible option would have been to create an inner class such as Part, but I didn't adopt it because it would require relatively many changes to the codebase.

Testing Instructions

Add an unit test in serialization package
Set up reproducible environment for the issue, and confirm that this PR fix that

# FastAPI's endpoint
@app.post("/upload")
async def upload(name: str = Form(...), file: UploadFile = File(...)) -> int:
    return file.size

OpenAPI Spec

```yaml { "openapi": "3.0.2", "info": { "title": "Custom title", "summary": "This is a very custom OpenAPI schema", "description": "Here's a longer description of the custom **OpenAPI** schema", "version": "3.0.2" }, "paths": { "/": { "get": { "summary": "Root", "operationId": "root__get", "responses": { "200": { "description": "Successful Response", "content": { "application/json": { "schema": { "type": "string", "title": "Response Root Get" } } } } } } }, "/upload": { "post": { "summary": "Upload", "operationId": "upload_upload_post", "requestBody": { "content": { "multipart/form-data": { "schema": { "$ref": "#/components/schemas/Body_upload_upload_post" } } }, "required": true }, "responses": { "200": { "description": "Successful Response", "content": { "application/json": { "schema": { "type": "integer", "title": "Response Upload Upload Post" } } } }, "422": { "description": "Validation Error", "content": { "application/json": { "schema": { "$ref": "#/components/schemas/HTTPValidationError" } } } } } } } }, "components": { "schemas": { "Body_upload_upload_post": { "properties": { "name": { "type": "string", "title": "Name" }, "file": { "type": "string", "format": "binary", "title": "File" } }, "type": "object", "required": [ "name", "file" ], "title": "Body_upload_upload_post" }, "HTTPValidationError": { "properties": { "detail": { "items": { "$ref": "#/components/schemas/ValidationError" }, "type": "array", "title": "Detail" } }, "type": "object", "title": "HTTPValidationError" }, "ValidationError": { "properties": { "loc": { "items": { "anyOf": [ { "type": "string" }, { "type": "integer" } ] }, "type": "array", "title": "Location" }, "msg": { "type": "string", "title": "Message" }, "type": { "type": "string", "title": "Error Type" } }, "type": "object", "required": [ "loc", "msg", "type" ], "title": "ValidationError" } } } } ```

microsoft / kiota-python