[IDEV-2584] RTTF Support for header authentication & CLI Improvements for download endpoint

[briluza]

Fix: RTTF API key no longer leaks as a plain query param.** When header_authentication is active (the default for all Real-Time Threat Feed endpoints), api_key is now sent exclusively via the X-Api-Key header and is removed from query parameters.

• Fix: endpoint=download no longer requires time params. validate_feeds_parameters was erroneously requiring sessionID/after/before for download calls. Download endpoints don't use a session window, so the check is now skipped when endpoint=download.

• Fix: download endpoint now returns correct result type and response shape. All 9 feed methods (nod, nad, noh, domainrdap, domaindiscovery, realtime_domain_risk, domainhotlist, iphotlist, iprisk) previously returned a streaming FeedsResults for the download path. They now return a standard Results with response_path=("response",), matching the actual JSON shape of the download API.

• Fix: feed-specific params no longer leak into download requests. Download path calls now only forward limit/page/prefix; stream-only params (sessionID, after, domain, etc.) are no longer included.

• CLI: added --limit to all 9 feed commands. Previously missing from the CLI surface; only applies to endpoint=download.

• Fix: CLI download output now renders as JSON. The download endpoint response was previously printed as a Python dict repr ({'key': ...}) because the formatter treated it as a streaming feed. It now correctly formats as JSON.

• Updated PYTHON_SUPPORT.md to reflect the Python >=3.9 requirement.

• Updated README.md Feeds section:

  • Corrected false claim that header_authentication defaults to False for download endpoints — it is True for all RTTF endpoints.
  • Added download endpoint usage example and note that time params are not required.
  • Added limit, page, prefix parameter documentation under a new "Download-only parameters" subsection.
  • Added note that top is ignored for the download endpoint.
  • Fixed typos: "RTUF" → "RTTF", "we may dealing" → "we may be dealing".

[jbabac]

Good job!