add Verbose.InfoSDepth

[pohly]

What this PR does / why we need it:

It is needed in Kubernetes for code like this (from
k8s.io/apiserver/pkg/server/httplog/httplog.go):

  // Log is intended to be called once at the end of your request handler, via defer
  func (rl *respLogger) Log() {
          ...

          klog.V(withLoggingLevel).InfoSDepth(1, "HTTP", keysAndValues...)
  }

Without InfoSDepth as method for Verbose, such code has to use klog.InfoSDepth,
which records the message with v=0 in JSON output. This is the wrong verbosity
for this debug message.

We don't need Verbose.ErrorSDepth (error messages have no verbosity) and also
no Verbose.Info[f]Depth (when touching code, it should be rewritten to use
Verbose.InfoSDepth instead)

Release note:

added Verbose.InfoS

[pohly]

I have changes ready for Kubernetes were I tentatively started using the new function.

[dims]

/approve
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dims, pohly

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