[CDAP-20795] remove identity from namespace wi request body

[itsankit-google]

Since the GcpWorkloadIdentityHttpHandler has access to the namespaceMeta we don't need the namespace identity in the request body.

Updated requests:

Create/Update Identity

PUT v3/namespaces/{namespace-id}/credentials/workloadIdentity

body: {"serviceAccount":"<GSA_EMAIL>"}

Validate Identity

POST v3/namespaces/<namespace>/credentials/workloadIdentity/validate

body: {"serviceAccount":"<GSA_EMAIL>"}