Add support for spire-server admin_ids

[marcofranssen]

See https://github.com/spiffe/spire/blob/main/doc/spire_server.md#server-configuration-file

Would be great if we somehow can configure the admin_ids.

E.g. via spire-controller-manager CRDs? Maybe as a separate Helm chart to provision admin_ids CRDs so we can configure the same on spire-server.