cephadm: improve oauth2-proxy validation error messaging

cephadm: improve oauth2-proxy validation error messaging

Previously, applying oauth2-proxy without required configuration
resulted in misleading field-level validation errors.

This change introduces:
- Early validation for missing spec in CLI path
- Aggregated missing field validation in OAuth2ProxySpec
- Clear and actionable error messages

Fixes:
- ceph orch apply oauth2-proxy without spec
- ceph orch apply -i <yaml> with missing required fields

Fixes: https://tracker.ceph.com/issues/76372
Signed-off-by: Shubha Jain <SHUBHA.JAIN1@ibm.com>