Merge pull request #37708 from juanvallejo/jvallejo/add-restclient-config-helper-for-parsing-global-timeout

Automatic merge from submit-queue (batch tested with PRs 37708, 34410)

Add restclientconfig helper fn for parsing timeout

Related downstream PR: https://github.com/openshift/origin/pull/12062 (example of use-case for this patch)

**Release note**:
```release-note
release-note-none
```

This patch adds a package `pkg/client/unversioned/clientcmd/util` and
defines a `ParseTimeout` helper function for parsing time from a
user-defined string. This allows code re-use in other packages that
require the creation of a new restclient (and therefore must set the
`--global-timeout` flag value manually).

@fabianofranz @kubernetes/cli-review