Automatic merge from submit-queue (batch tested with PRs 64057, 63223, 64346, 64562, 64408). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

kubeadm: Refactor the Bootstrap Tokens usage in the API types

**What this PR does / why we need it**:
This PR:
 - Moves some common, generic Bootstrap Token helpers and constants from `k8s.io/kubernetes/cmd/kubeadm/app/util/token` to `k8s.io/client-go/tools/bootstrap/token/`
 - Breaks out the top-level Bootstrap Token fields to a dedicated `BootstrapToken` struct with helper functions.
 - Instead of representing the Bootstrap Token as a plain `string`, there is now a wrapper struct `BootstrapTokenString` that can marshal/unmarshal correctly and supports validation on create, and splitting up the full token in the ID/Secret parts automatically.
 - Makes kubeadm support multiple Bootstrap Tokens automatically by supporting a slice of `BootstrapToken` in the `MasterConfiguration` API object
 - Consolidates the place for kubeadm to create token-related flags in an `options` package
 - Supports automatic conversion from the `v1alpha1` to `v1alpha2` API
 - Adds support to set token expiration directly instead of setting a TTL (Expiration and TTL are mutually exclusive)
 - Removes the old `TokenDiscovery` struct we're not using anymore inside of kubeadm

**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Related to https://github.com/kubernetes/community/pull/2131

**Special notes for your reviewer**:
This is work in progress. Please only review the first two commits for now.
I will work on splitting up this PR in smaller chunks.
I will also write unit tests tomorrow.

**Release note**:

```release-note
[action required] kubeadm: The Token-related fields in the `MasterConfiguration` object have now been refactored. Instead of the top-level `.Token`, `.TokenTTL`, `.TokenUsages`, `.TokenGroups` fields, there is now a `BootstrapTokens` slice of `BootstrapToken` objects that support the same features under the `.Token`, `.TTL`, `.Usages`, `.Groups` fields.
```
@kubernetes/sig-cluster-lifecycle-pr-reviews @mattmoyer @liztio