[New RM] Add duckduckgo retriever

[kosiew]

Add retriever DuckDuckGoSearch

DuckDuckGoSearch is a free web search API that does not require an API key.

--retriever will accept duckduckgo

eg

python examples/run_storm_wiki_gpt.py --retriever duckduckgo --do-research --do-generate-outline --do-generate-article --do-polish-article --remove-duplicate --max-perspective 5 --max-thread-num 5 --output-dir $OUTPUT_DIR

[kosiew]

Sorry for the formatting diff.

I used yapf and .style.yapf

[style]
based_on_style = pep8
column_limit = 79
# Add any other configurations you prefer here

but it still introduced a lot of formatting diff.

[shaoyijia]

Thank you so much for working on this!

but it still introduced a lot of formatting diff. We already know this issue. We will set up a formatter to make the contribution less painful. For this PR, could you manually revert change on parts unrelated to DuckDuckGoSearch? This could help us merge it. Thanks!

[shaoyijia]

Close it because DuckDuckGo has been supported. Thanks for working on this though!