Kubeseal to use cobra library

**Which component**:

- kubeseal

**Is your feature request related to a problem? Please describe.**

The current CLI, kubeseal, is quite complex since every action this CLI can perform depends on a series of flags. In other words, there are no subcommands to help simplifying the CLI usage for certain use cases.

You can simply run the command below to realize there are **47 supported flags**!!

```console
$ kubeseal --help
Usage of kubeseal:
(...)
```

This result in a poor UX that makes it quite difficult to new potential users to adopt the project.

**Describe the solution you'd like**

Use [cobra library](https://github.com/spf13/cobra) to split the current supported use cases on several subcommands. For instance:

- `kubeseal fetch` in replacement of `--fetch-cert` flag.
- `kubeseal merge` in replacement of `--merge-into` flag.
- `kubeseal re-encrypt` in replacement of `--re-encrypt` flag.
- `kubeseal recovery` in replacement of `--recovery-unseal` flag.
- `kubeseal raw` in replacement of `--raw` flag.
- `kubeseal validate` in replacement of `--validate` flag.

Cobra also makes it very simple to create/manage a local config file for kubeseal (`~/.kubeseal/config`) and create a `kubeseal config` subcommand to allow users to configure locally settings such as the controller name and the controller namespace. This can also help to reduce the number of required flags to use the CLI.

**Describe alternatives you've considered**

N/A

**Additional context**

Adopting the cobra interface is an important code refactoring that will require adapting all the docs and it'll require bumping the major version. That said, we should prioritize not introducing incompatibilities in the controller side with kubeseal `0.x` series.