Auth: Add missing request headers to SigV4 middleware allowlist (#30115) (#30270)

* Auth: Add Content-Type to SigV4 header allowlist

* add MT headers to permitted headers

* add Kibana API header to allowlist

* add Content-Length header to allow list

(cherry picked from commit a0a1422b2d)

Co-authored-by: Will Browne <wbrowne@users.noreply.github.com>

pkg/models/sigv4.go

@@ -33,6 +33,11 @@ var permittedHeaders = map[string]struct{}{
 	"User-Agent":      {},
 	"Accept":          {},
 	"Accept-Encoding": {},
+	"Content-Type":    {},
+	"Content-Length":  {},
+	"securitytenant":  {},
+	"sgtenant":        {},
+	"kbn-xsrf":        {},
 }
 
 type SigV4Middleware struct {