Switch calls to use snake_case instead of camelCase

[BScong]

Following modifications on API, we need to switch some variables from camelCase to snake_case

Here are the modifications made on the API side :

• The extendedData body parameter becomes extended_data (used on /intent:ScoutTitles, /intent:SearchAndPlayArticle, /intent:SearchAndSummarizeArticle, /article, /summary, /search)

• The searchTerms body parameter becomes search_terms (used on /intent:SearchAndPlayArticle and /intent:SearchAndSummarizeArticle)

• The lengthMinutes value becomes length_minutes (on every endpoint returning article data)

• The imageURL value becomes image_url(on every endpoint returning article data)

Both syntaxes (camelCase and snake_case) will coexist until all changes are made on the apps. Then, we will delete the camelCase variables.