Adding fields to add values in headers

I didn't run a `diff`, but at a quick glance, the patch and the MR look the same. Let's work in the MR though? Patches can just be for Composer.

+++ b/src/Plugin/ExternalEntities/StorageClient/Rest.php
@@ -180,41 +170,57 @@ class Rest extends ExternalEntityStorageClientBase implements PluginFormInterfac
+      $sequence[$exploded[0]] = $value;

Would it make sense to add a Token::replacePlain() in here to expand tokens in values?

In our case, users will each have an ID that they pass in for authentication & authorization so it'll be something like [User:remoteID].

On v3, this is currently handled for a single custom header (ie. "Custom header" option in the "Authentication" section). Do you need more than one header item or could some of them be passed to the other section "HTTP protocol > Additional HTTP headers"?

Should an event be added to support more header customization?