Allow to define HTTP headers to XWiki extension repositories

In the same way as XCOMMONS-2099, adding custom HTTP headers to XWiki-based extension repositories could also be helpful in order to integrate with wikis having OIDC-based authentication, where the authentication can be done through HTTP Headers.

As for Maven-based extension repository, the HTTP headers configuration of an XWiki extension repository could be done using the following configuration key :

extension.repositories.MY_REPOSITORY_ID.http.headers.MY_HEADER_NAME