datahub-frontend chart OIDC support incomplete

[jdnurmi]

Describe the bug
The default chart only allows a few hard-coded OIDC providers (oidcAuthentication.providers)

Expected behavior
Any OIDC provider should work (within some bounds of normal operations)

Additional context

For my clients configuration, I did a micro patch:

diff --git a/charts/datahub/subcharts/datahub-frontend/templates/deployment.yaml b/charts/datahub/subcharts/datahub-frontend/templates/deployment.yaml
index b45e53e..45f9d69 100644
--- a/charts/datahub/subcharts/datahub-frontend/templates/deployment.yaml
+++ b/charts/datahub/subcharts/datahub-frontend/templates/deployment.yaml
@@ -242,6 +242,11 @@ spec:
               value: https://login.microsoftonline.com/{{ .azureTenantId }}/v2.0/.well-known/openid-configuration
             - name: AUTH_OIDC_SCOPE
               value: {{ .scope | default "openid profile email" }}
+            {{- else if .issuerUrl }}
+            - name: AUTH_OIDC_DISCOVERY_URI
+              value: {{ .issuerUrl }}/.well-known/openid-configuration
+            - name: AUTH_OIDC_SCOPE
+              value: {{ .scope | default "openid profile email" }}
             {{- else }}
             {{- fail (printf "unsupported .oidcAuthentication.provider value '%s'" .provider) }}
             {{- end }}

Which allowed me to enable an arbitrary provider (in our case, OneLogin). There may be better ways to achieve this, so I defer to the chart-keepers on their preferred solution - but in case someone else comes looking, hopefully they'll find this.