[dart] fix: Bearer and Basic auth interceptors case-insensitive scheme comparison (17073)

[karadzhov]

Solves #17073

@agilob @jaumard @josh-burton @amondnet @sbu-WBT @kuhnroyal @agilob @ahmednfwela

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[kuhnroyal]

LGTM thanks!

[kuhnroyal]

@wing328 Can you merge this please.