[Proposal] Add ability to read authorized keys from shell script instead of file

[Constantine]

File authorized_keys is unusable for mass key storage and manipulation. I
wan to store keys in something like mysql server, but It will add big
unwanted dependency to package. What if we use auth_rsa.c but instead
search in file send key to some script and read sigle return value if key
finded and empty if not. I think it will be very customizable.

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With Best Regards,
Constantine