feat: 🎸 Make adding client_id to requesterID configurable

Configurable - if the property saml.requester-id.prefix is available, the client ID will be added (prefixed with the value of the property) to the RequesterIDs in SAML. For example, if the clientID is APP-123 and the property is set to saml.requester-id.prefix=urn:geant:proxy, the resulting SAML RequesterID element will have the value urn:geant:proxy:APP-123. The value of the property should not end with a colon character. To disable passing clientID via SAML RequesterID element, leave the configuration option empty.