Licensing: Send map of environment variables to plugins (#30347)

Replace hard-coded `GF_ENTERPRISE_LICENSE_TEXT` with a
`map[string]string` to allow the licensing service to allow sending any
environment variable through to the plugins for licensing purposes.

pkg/plugins/backendplugin/manager_test.go

@@ -411,6 +411,6 @@ func (t *testLicensingService) HasValidLicense() bool {
 	return false
 }
 
-func (t *testLicensingService) TokenRaw() string {
-	return t.tokenRaw
+func (t *testLicensingService) Environment() map[string]string {
+	return map[string]string{"GF_ENTERPRISE_LICENSE_TEXT": t.tokenRaw}
 }