Read env to change the CSR signer name

[jchanam]

What type of PR is this?
/kind feature

What this PR does / why we need it:
As stated on the README, on EKS 1.22 it's needed to use the certificate signer beta.eks.amazonaws.com/app-serving.

The issue is that with the current code, the only way to do it is changing it, creating a new and custom docker image, and use it with the new signer.

This PR adds a new environmental variable that allows to define this signer to the one the user wants, with the default value that it's currently being used (kubernetes.io/kubelet-serving), so no breaking changes are introduced.

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

New env `IMAGESWAP_CSR_SIGNER_NAME` added, allowing to define what signer to use on the Certificate Signing Request.

Additional documentation e.g., usage docs, etc.:

On the current deploy files, a new element should be added to the 'imageswap-env' configmap, resulting in:

kind: ConfigMap
apiVersion: v1
metadata:
  name: imageswap-env
  namespace: imageswap-system
  labels:
    app: imageswap
    resource: configmap
data:
  FLASK_ENV: "production"
  PYTHONUNBUFFERED: "TRUE"
  IMAGESWAP_MODE: "MAPS"
  IMAGESWAP_LOG_LEVEL: "INFO"
  IMAGESWAP_CSR_SIGNER_NAME: "beta.eks.amazonaws.com/app-serving"

[phenixblue]

Looks good for the most part, just a simple docs update and we should be good to go.

[phenixblue]

/lgtm