Custom HTML on successful login page

[bartlettc22]

Adding a CLI option for adding a custom HTML string for the successful login page. I saw you already had this option in the oauth2cli package so I brought in through to kubelogin.

Fixes #307

[codecov-commenter]

Codecov Report

Merging #308 into master will increase coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #308      +/-   ##
==========================================
+ Coverage   72.37%   72.48%   +0.11%     
==========================================
  Files          20       20              
  Lines         713      716       +3     
==========================================
+ Hits          516      519       +3     
  Misses        130      130              
  Partials       67       67              

Impacted Files	Coverage Δ
pkg/usecases/authentication/authentication.go	83.72% <ø> (ø)
pkg/adaptors/cmd/root.go	91.17% <100.00%> (+0.26%)	⬆️
pkg/usecases/authentication/authcode.go	64.00% <100.00%> (+0.73%)	⬆️

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[sfxworks]

How does one apply this?

- name: keycloak
  user:
    exec:
      apiVersion: client.authentication.k8s.io/v1beta1
      args:
      - oidc-login
      - get-token
      - --oidc-issuer-url=https://keycloak.sfxworks.net/auth/realms/mcsh
      - --oidc-client-id=account
      - --oidc-client-secret=my-secret
      - --server-success-html='<iframe src="https://my-iframe.example" title="Welcome"></iframe>'
      command: kubectl
      env: null

Yields

$ kubectl get pods
error: unknown flag: --server-success-html
exit status 1
error: unknown flag: --server-success-html
exit status 1

on build.

Yet the option is available in kubelogin.