mgr/cephadm: introducing new cmd to generate self-signed certs

mgr/cephadm: introducing new cmd to generate self-signed certs

this new Cephadm cmd introduces the ability to generate self-signed
certificates for external modules, signed by Cephadm as the root CA.
This feature is essential for implementing mTLS. Previously, if the
user did not provide a certificate and key, the dashboard would
generate its own. With this update, the dashboard now calls Cephadm
to generate self-signed certificates, enabling secure mTLS
communication with other backend applications. Prometheus module
also makes use of this new functionality to generate self-signed
certificates.

Signed-off-by: Redouane Kachach <rkachach@ibm.com>