The struct is included in InitConfiguration, JoinConfiguration
and ResetConfiguration.

Add conversion and update defaulters and fuzzers.
Include a timeoututils.go that contains a function
to default the internal Timeouts struct.

When upconverting from v1beta3 to v1beta4, it appears there is no
easy way to migrate some of the timeout values such as:
  ClusterConfiguration.APIServer.TimeoutForControlPlane
to a new location:
  InitConfiguration.Timeouts.<some-timeout-field>

Yes, the internal InitConfiguratio does embed a ClusterConfiguration,
but during conversion the ClusterConfiguration is converted from an
empty source.

K8s' API machinery has ways to register custom conversion functions,
such as v1beta3.ClusterConfiguration -> internal.InitConfiguration,
but these must be triggered explicitly with a decoder.
The overall migration of fields seems very awkward.

There might be hacks around that, such as storing intermediate state,
while trying to make the fuzzer rountrip happy, but instead
mutation functions can be implemented for the internal types when
calling kubeadm's migrate code. This seems much cleaner.

Currently, timeouts are only accessible if a kubeadm runtime.Object{}
like InitConfiguration is passed around.

Any time a config is loaded or defaulted, store the Timeouts
structure in a thread-safe way in the main kubeadm API package
with SetActiveTimeouts(). Optionally, a deep-copy can be
performed before calling SetActiveTimeouts(). Make this struct
accessible with GetActiveTimeouts(). Ensure these functions
are thread safe.

On init() make sure the struct is defaulted, so that unit
tests can work with these values.

Replace the usage of the deprecated wait.Poll() and
wait.PollImmediate() functions with wait.PollUntilContextTimeout().
Since we don't have piping of context around kubeadm,
use context.Background() everywhere.

Some wait.Poll() functions were converted to "immediate" as there
is no point for them to not be. This is done for consistency.

Replace the only instance of wait.JitterUntil with
wait.PollUntilContextTimeout. JitterUntil is not deprecated
but this is also done for consistency.

Propagate usage of the Timeout struct values.
Apply sanitazation to timeout constants in contants.go.

Switch to PollUntilContextTimeout() everywhere to allow
usage of the exposed timeouts in the kubeadm API. Exponential backoff
options are more difficult to expose in this regard and a bit too
detailed for the common user - i.e. have "steps", "factor" and so on.