Allow name in omniauth providers

What does this MR do?

Currently, omniauth providers configuration must be put in a Secret whose name is then referenced in the chart values, this is the sensible thing to do since these settings include secrets.

But in the case of a few providers, this is not the case: group_saml and kerberos. For both providers, the only configuration needed is their name. Instead of having to create secrets such as this one:

apiVersion: v1
kind: Secret
metadata:
  name: gitlab-provider-kerberos
stringData:
  provider: |
    name: kerberos

omniauth:
  providers:
    - secret: gitlab-provider-kerberos

We could allow the user to set directly the provider name into the providers list:

omniauth:
  providers:
    - name: kerberos
    - name: group_saml

This MR also allow setting the optional label and icon parameters.

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.