feat: Implement a backend endpoint that handles the upload of compliance certificates in PDF format

[borjakhet]

WHAT

This PR implements a new backend endpoint POST /v1/addons/ccm-kit/certificates/ that handles the upload of compliance certificates in PDF format. The backend is responsible for handling the binarized PDF, enriching it with metadata, and persisting both the file and its metadata in the local PostgreSQL database.

The endpoint expects an object with the following content:

{
  "certificate_type": "CERTIFICATE_TYPE",
  "certificate_name": "CERTIFICATE_NAME",
  "issuer_or_certification": "CERTIFICATE_ISSUER",
  "valid_from": "DATE_FROM",
  "valid_to": "DATE_TO",
  "bpn": "BPN_NAME",
  "description": "CERTIFICATE_DESCRIPTION",
  "doc": "ENCODED_PDF_IN_BINARY_FORMAT"
}

Closes #476

[matbmoser]

@jalvaro-lks please check this pr and approve it, test your frontend features with this PR content.