Allow file uploads

[dontub]

This change allows to upload files in remote entity forms. Files have to be transmitted in the following form in the form data:

{
  "filename": <string>,
  "content": <Base64 encoded file>
}

In the JSON schema existing files are specified as default in the following form:

{
  "filename": <string>,
  "url": <URL to download the file>
}

Because of the Base64 encoding a possible maximum file size is only validated approximately.
See FileFieldFactory.

There are two TODOs:

1. What to do with previous files? https://github.com/systopia/de.systopia.remotetools/pull/49/files#diff-2a75cd2f53711c0051277a95edc7a2ae42bb31413045d54074cbab774d4c2ff9R66
2. It's not possible to set a custom file field to NULL. https://github.com/systopia/de.systopia.remotetools/pull/49/files#diff-2a75cd2f53711c0051277a95edc7a2ae42bb31413045d54074cbab774d4c2ff9R99 (Not necessary for the current use case.)

How should we treat them?

systopia-reference: 25860