Kubeseal as a krew-plugin

**Which component**:
kubeseal

**Is your feature request related to a problem? Please describe.**
Managing local tools related to Kubernetes was a bit of a hazzle until I found krew, the kubectl plugin package manager. After that I had a slight hope that I could use Krew to manage all the tools I need for kubectl through the same plugin system (since the kubectl plugin system is quite versatile).

**Describe the solution you'd like**
Deploy a krew-manifest to the community index that allows for easy installation for anyone that has krew.
Something that allows for `kubectl seal -f -w `.

**Describe alternatives you've considered**
I uses homebrew today.

**Additional context**
There might be several technical and non-technical reasons why this is undesired, unwise, or even stupid, but I found krew to be neat, and was disappointed that kubeseal was not available in the krew index since it felt appropriate that it is a kubectl-plugin 😄

The issue names no repository files or tests. Start by checking Krew's community index contribution requirements and how kubeseal is packaged; done means a Krew installation can provide the requested kubectl seal command for the stated input and output files.

Written by the indexing model from the issue text.