[BUG] CustomParser request header issue when sending json and file

[BQBB]

Bug description
The challenge encountered arises within the CustomParser during the instantiation of the parse_body component. This occurs when handling requests that may include both file uploads and JSON payloads. The request is processed utilizing an instance from _HttpRequest. However, a noted limitation surfaces in this context; the class lacks provisions for header accessibility.

In summary, the primary concern lies with extracting headers from within the _HttpRequest class during the parsing procedure. This highlights a requirement to either introduce mechanisms to retrieve headers or adjust the current workflow to accommodate this constraint.

if this is considered a limitation, it would be my delight to address this limitation in a pull request.

Versions:

• Python version: 3.11.7
• Django version: 4.2.5
• Django-Ninja version: 1.1.0