Changes saltSeparator type from string to Buffer

[ggirotto]

Fix an inconsistency where hash.saltSeparator on UserImportOptions is expecting a Buffer but its type on Typescript module was specified as a string

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[ggirotto]

@googlebot I signed it!

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CLAs look good, thanks!

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[ggirotto]

Hi @bojeil-google. There's something else that I have to do or just wait for the merge?