fix: remove embedded-policy as requested by the user (#14847)

this PR introduces a few changes such as

- sessionPolicyName is not reused in an extracted manner
  to apply policies for incoming authenticated calls,
  instead uses a different key to designate this
  information for the callers.

- this differentiation is needed to ensure that service
  account updates do not accidentally store JSON representation
  instead of base64 equivalent on the disk.

- relax requirements for Deleting a service account, allow
  deleting a service account that might be unreadable, i.e
  a situation where the user might have removed session policy 
  which now carries a JSON representation, making it unparsable.

- introduce some constants to reuse instead of strings.

fixes #14784

cmd/auth-handler.go

@@ -235,7 +235,7 @@ func getClaimsFromTokenWithSecret(token, secret string) (map[string]interface{},
 			logger.LogIf(GlobalContext, err, logger.Application)
 			return nil, errAuthentication
 		}
-		claims.MapClaims[iampolicy.SessionPolicyName] = string(spBytes)
+		claims.MapClaims[sessionPolicyNameExtracted] = string(spBytes)
 	}
 
 	return claims.Map(), nil