Add a mechanism for configuring OaiClients to send custom http headers with OAI requests

[landreev]

This is to accommodate IQSS/dataverse/issues/9231. This can be achieved by simply making the class io.gdcc.xoai.serviceprovider.client.JdkHttpOaiClient non-final, then extending it with the extra functionality on the Dataverse side. But based on a quick discussion in the issue above, it would make more sense to add it to the xoai code here. Will make a PR shortly.