[Tour Of Beam] delete user progress

[eantyshev]

resolves #24659

authorized CF endpoint postDeleteProgress deletes all tb_user & tb_user_progress entities related to a user
postDeleteProgress is idempotent by uid (derived from firebase ID token), which makes it possible to retry
README updated

++ small refactoring of middlewares make them less coupled (store SDK and UID in request context, not in arguments)

Should be followed by user deletion via Firebase API on the frontend side.

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[olehborysevych]

LGTM

[damccorm]

LGTM - thanks!