Vault storage: Support kubernetes auth method about lua-resty-acme

[fffonion]

@UXabre The WIP code looks awesome!
I'm not super familar with how to test upon k8s world without bring up
real k8s cluster (minikube?). But that may be too compilcated and make the test takes much more time to run.
Actually I think if you can capture what a real Vault agent sent when it's authenticating to Vault server, and use that
as an "expected result", it's kind of acceptable as well.

[UXabre]

I'll try doing it using a JWT auth method. It behaves very similar to the kubernetes auth (only very minor changes, an nothing which affects the code). It, at least, is much more lightweight then setting up k8s/k3s.

I'm currently running the test suites and it will probably need a bit of rinse and repeat before I'll make the PR... nevertheless, I hope to have it in a few hours.

[fffonion]

Resource for developer for this feature: https://www.vaultproject.io/docs/auth/kubernetes

[UXabre]

Thanks for the input!
I'm currently working on implementing this.
I'm struggling a bit to get this automatically tested tough...

Here's my WiP so far:
UXabre@81de6f9

[UXabre]

I've made it! The automated test is running; but I was unsure where to put the prepping of Vault (as to include JWT auth etc.), I've added it to tests.yml in the github workflow directory