ssh/knownhosts: add support for parsing known hosts key metadata

[based64god]

This patch includes a new API for consumers to access their known_hosts' keys, as well as relevant metadata included in the existing KnownHost.

[gopherbot]

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

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Patch Set 1:

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[gopherbot]

This PR (HEAD: 8963009) has been imported to Gerrit for code review.

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[gopherbot]

This PR (HEAD: a8e546a) has been imported to Gerrit for code review.

Please visit Gerrit at https://go-review.googlesource.com/c/crypto/+/621335.

Important tips:

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