crypto/x509: add SignatureAlgorithm.Hash()

[bodgit]

Add method to return the hashing algorithm associated with a
certificates signature algorithm. This is useful when generating TLS
channel bindings as documented in RFC 5929.

Fixes #33317

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Message from Go Bot:

Patch Set 1:

Congratulations on opening your first change. Thank you for your contribution!

Next steps:
Within the next week or so, a maintainer will review your change and provide
feedback. See https://golang.org/doc/contribute.html#review for more info and
tips to get your patch through code review.

Most changes in the Go project go through a few rounds of revision. This can be
surprising to people new to the project. The careful, iterative review process
is our way of helping mentor contributors and ensuring that their contributions
have a lasting impact.

During May-July and Nov-Jan the Go project is in a code freeze, during which
little code gets reviewed or merged. If a reviewer responds with a comment like
R=go1.11, it means that this CL will be reviewed as part of the next development
cycle. See https://golang.org/s/release for more details.

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Please don’t reply on this GitHub thread. Visit golang.org/cl/187778.
After addressing review feedback, remember to publish your drafts!

[bodgit]

@heschi Was there any feedback?

[heschi]

I closed old PRs to reduce load on the Gerrit importer (#50197), sorry for the trouble. I'll reopen the CL and PR.