[Platform] Make client cert optional in custom certs workflow

[iSignal]

After the work related to #7153 (comment) we no longer need a client cert by default to connect to a TLS enabled universe. We need to simplify our custom certs flow to make the client cert part optional. When the user creates an onprem universe that points to a custom cert and checks "client to node encryption in transit", we should not fail the operation if the custom cert does not contain a client cert.

[sshev]

@hkandala the client cert field at Add Certificate modal is already optional in UI
let me know if there are any additional changes required on UI

[hkandala]

No changes are needed from backend too.
Verified the code flow and client certificates are considered optional everywhere.
Also verified by creating on-prem universe with client to node encryption enabled using custom certificates (without client cert). Was able to successfully create and connect to the universe.