Align RP ID string types

[emlun]

rp.id in PublicKeyCredentialCreationOptions and rpId in PublicKeyCredentialRequestOptions represent the same thing, but with different types. The WG agreed on the 2024-05-15 call that both should be USVString. Strictly speaking this is a breaking change (changing a type bound in input (contravariant) position to be more restrictive), but in practice this shouldn't be able to break any applications since then those credentials wouldn't have worked in get() anyway.

Fixes #2066.

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