Allow users to reference own public key secrets

[puffitos]

Motivation

This should solve #17

Changes

I've extended the secret parsing function to check for the referenced secret (as was the original intention, when reading the docs). The default secret still works as a fallback.

To achieve this, I moved the client to the webhook server itself, to avoid uneccesary creation of multiple k8s clientsets. One is enough to handle the requests we get.

Additionally, I've cleaned up the code base a bit, removed the vendors folder (if there is some reason for it, we can leave it in) and spruced up the documentation.

Tests done

System tests for the following use cases:

1. Wrong public key present in secret -> no deployment
2. Wrongly encoded key present in secret -> no deployment
3. No referenced secret present at all, no env variables, no default namespaced secret -> deployment
4. Referenced secret key is empty -> deployment (if all other checks pass)

Also added some unit tests to get the ball rolling :)

[puffitos]

OMG Github will hate me for this 🗡️

[mamofejo]

would it be possible to use an annotation instead of an environment variable for the name of the secret? This would not pollute the container environment.

[puffitos]

This is a interesting suggestion, but it doesn't really fit the scope of this PR. We're correcting the expected behavior here: one should be able to reference an own secret and not only the default one. The feature is suggested by the repo's Readme but isn't actually there. 😉

Still, I do encourage you to open an issue so we may discuss a possible implementation.