Fix types for Create / Update MultifactorRequest

[jakebiesinger-storyhealth]

Fixes #1267.

Derived classes are not interchangeable with Base classes as types. Rather than making the request take a generic <T extends CreateMultifactorInfoRequest>, this assigns a union of the (one currently allowed) type.

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[jakebiesinger-storyhealth]

@googlebot I signed it!

[hiranya911]

Superseded by #1294