Add derivations function to the keystore metadata

[ph]

When we save the Keystore to disk, we also preserve some metadata required to decrypt the file. Currently, we store the following information:

VERSION|SALT|IV|PAYLOAD

As suggested in this comment it would make sense also to store the name of the derivation function we are currently using.

This change makes it easier to swap the derivation function without increasing the version number.

[ph]

I will hold on this until #6016 (comment) is stabilized across the stack.

[botelastic]

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[botelastic]

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