Add an event for setting authentication headers in a GET request made by `choiceByUrl`

[JaneSjs]

User issues

• T16070 - Set headers Authentication in ChoiceByUrl https://surveyjs.answerdesk.io/internal/ticket/details/T16070
• T14803 - How could we consume different private APIs (Authenticated APIs) for dropdown. https://surveyjs.answerdesk.io/internal/ticket/details/T14803
• T15766 - Survey PDF- How to get variable values in form? https://surveyjs.answerdesk.io/internal/ticket/details/T15766 ...and more

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https://github.com/surveyjs/survey-library/blob/38bf35cd7667f16e0567bfe7081bb4d017dba5fc/src/choicesRestful.ts#L106C17-L106C36

[RomanTsukanov]

settings.web.onBeforeRequestChoices