Add FormData and multipart/form-data support

[zoedsoupe]

Problem

JS client supports FormData for file uploads. Elixir only supports Map (JSON), String, and Binary.

Proposed Solution

multipart = [
  {:file, binary, filename: "doc.pdf", content_type: "application/pdf"},
  {:field, "name", "document"}
]
Supabase.Functions.invoke(client, "upload", body: {:multipart, multipart})

Benefits

• File upload support
• Feature parity with JS client
• Better Phoenix/Plug integration

JS Reference: functions-js/src/FunctionsClient.ts:142