Accepts multi-dimensional arrays as value for formdata field value

[RomainMazB]

Description
As of today, each part of a request using the multipart option should either:

• be a resource
• be a string
• implements StreamInterface

But a lot of users naturally expect to be able to pass an array, issues are created since 2015 with this natural expectation:

• Multipart with an array form value?
• Send files and multidimensional array with Guzzle 6
• How to send duplicate keys in post data?
• How to pass multidimensional Array and multi files ?

Example

$client->request('POST', '/post', [
  'multipart' => [
    [
      'name' => 'products',
      'contents' => [
        ['id' => 123, 'quantity' => 1],
        ['id' => 456, 'quantity' => 2],
        ['id' => 789, 'quantity' => 3]
      ]
    ]
]);

Additional context
I faced this issue when using Guzzle through the Laravel Http facade.
I tried to fix this from the fw side but Taylor Otwell suggested to fix this inside Guzzle directly.

I continued to investigate on this on both Guzzle, Laravel and even other fw way to tackle this, so I am able to do a PR to add this feature.
Basically it would expand the array values into multiple parts, the below request would results to this one:

$client->request('POST', '/post', [
  'multipart' => [
    [
      'name' => 'products[0][id]',
      'contents' =>'123'
    ],
    [
      'name' => 'products[0][quantity]',
      'contents' =>'1'
    ],
    [
      'name' => 'products[1][id]',
      'contents' =>'456'
    ],
    [
      'name' => 'products[1][quantity]',
      'contents' =>'2'
    ],
    [
      'name' => 'products[2][id]',
      'contents' =>'789'
    ],
    [
      'name' => 'products[2][quantity]',
      'contents' => '3'
    ]
]);