feat(controller): support receiving MultipartFiles

[cromoteca]

Multipart requests are supposed to contain some files (with name) and one JSON part named "hilla_body_part", which contains the ordinary data.

The part names for files must be named according to the JSON syntax for accessing a JSON object. So, for example, if a method has a parameter named uploadedFile, the corresponding path will be /uploadedFile. But paths can also point anywhere in the object, like /orders/1/invoice.

Closes #3130

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.65%. Comparing base (6de563d) to head (b5e645c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3165   +/-   ##
=======================================
  Coverage   92.65%   92.65%           
=======================================
  Files          85       85           
  Lines        3187     3187           
  Branches      778      778           
=======================================
  Hits         2953     2953           
  Misses        182      182           
  Partials       52       52           

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[cromoteca]

To be clear, this also works for top-level parameters:

public String uploadAvatar(String user, MultipartFile file) {
    return String.format("The avatar for %s is %d bytes.\n", user, file.getSize());
}

Request follows:

POST http://localhost:8080/connect/HelloEndpoint/uploadAvatar HTTP/1.1
Host: localhost:8080
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:134.0) Gecko/20100101 Firefox/134.0
Accept: application/json
Accept-Language: en-GB,en;q=0.8,fr-FR;q=0.6,fr;q=0.4,it;q=0.2
Accept-Encoding: gzip, deflate, br, zstd
Content-Type: multipart/form-data;boundary=----MYBOUNDARY
Referer: http://localhost:8080/
X-CSRF-Token: e44c315f-b825-4057-ba5f-2967ed167c71
Origin: http://localhost:8080
DNT: 1
Sec-Fetch-Dest: empty
Sec-Fetch-Mode: cors
Sec-Fetch-Site: same-origin
Connection: keep-alive
Cookie: Idea-ca342cc6=93254400-1659-49ba-9b12-282fb6325c2b; JSESSIONID=04D782331CAE64FB18EED57A754B7661; csrfToken=e44c315f-b825-4057-ba5f-2967ed167c71
Priority: u=0
Pragma: no-cache
Cache-Control: no-cache

------MYBOUNDARY
Content-Disposition: form-data; name="data"
Content-Type: application/json

{
  "user": "John"
}
------MYBOUNDARY
Content-Disposition: form-data; name="/file"; filename="avatar.png"
Content-Type: image/png

iVBORw0KGgoAAAANSUhEUgAAACAAAAAgCAMAAABEpIrGAAAABGdBTUEAALGPC/xhBQAAAAFzUkdCAdnJLH8AAAAgY0hSTQAAeiYAAICEAAD6AAAAgOgAAHUwAADqYAAAOpgAABdwnLpRPAAAAAZQTFRFAAAA/wAAG/+NIgAAAAF0Uk5TAEDm2GYAAABsSURBVDjLvZNLCgAhDEOT+196GBgY2yYGXJhl+qz9KNDFV/DiJ+GXeCV+jxJYTEqCWil+ATA1yk67M4h4AptbiyHLkglaCgYgdJ4BbgfDMcc+2rEJjJTc6GSZ4c0aALY++6vOAaS4/7oNWY0H7vQCVCaU9zAAAAAASUVORK5CYII=
------MYBOUNDARY--

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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