ssh: report verified signature format to callback

[Daquisu]

VerifiedPublicKeyCallback currently receives the public-key algorithm
declared in the outer authentication request. For RSA authentication,
that value can differ from the format of the signature that was
successfully verified because compatible RSA algorithm combinations are
accepted.

This can cause post-verification policy or audit decisions to use the
request algorithm instead of the signature format actually verified.

Pass sig.Format to VerifiedPublicKeyCallback so signatureAlgorithm
identifies the successfully verified signature format. Clarify the
callback documentation and add a regression test covering compatible
but different RSA request algorithms and signature formats.

Fixes golang/go#80411

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Patch Set 2:

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Patch Set 2: Code-Review+2

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This PR is being closed because golang.org/cl/800740 has been merged.