Support multipart/form-data

[hatamiarash7]

Description

After this PR, we can send POST requests in multipart/form-data format. For example, users can test the upload process quickly.

http_post_body_multipart:
    prober: http
    timeout: 5s
    http:
      method: POST
      body_multipart:
        - type: "file"
          key: "file"
          value: "/files/body.txt"
        - type: "text"
          key: "name"
          value: "arash"

We can define text or file fields here.

---

Changes

• Support multipart for HTTP probe
• Update example file
• Update configuration document

[hatamiarash7]

any update?