Implement missing IamClientAuthMethods for technical user creation

[Phil91]

Description

It would make sense to implement other IamClientAuth Methods for the technical user creation for endpoint POST: api/administration/serviceaccount/owncompany/serviceaccounts

Acceptance Criteria

Technical User Creation is implemented for Methods:

JWT,
X509,
SECRET_JWT

[evegufy]

@Phil91 is this issue still relevant?

[Phil91]

@evegufy one could still do it, we don't necessary need it. but would give more flexibility