add rekor APP to arkade

[developer-guy]

Similar to #488, rekor has a helm chart, we can use that to install rekor. https://github.com/sigstore/helm-charts/tree/main/charts

Expected Behaviour

$ arkade install rekor

Current Behaviour

There is no support for installing rekor app right now.

Are you a GitHub Sponsor (Yes/No?)

Check at https://github.com/sponsors/alexellis

• [x] Yes
• [ ] No

Possible Solution

Steps to Reproduce (for bugs)

Context

Your Environment

• What Kubernetes distribution are you using?

kubectl version

• Operating System and version (e.g. Linux, Windows, MacOS):

uname -a

cat /etc/os-release

• What arkade version is this?

arkade version