Adding support for backend plugin client middlewares. This allows headers in outgoing
backend plugin and HTTP requests to be modified using client middlewares.
The following client middlewares added:
Forward cookies: Will forward incoming HTTP request Cookies to outgoing plugins.Client
and HTTP requests if the datasource has enabled forwarding of cookies (keepCookies).
Forward OAuth token: Will set OAuth token headers on outgoing plugins.Client and HTTP
requests if the datasource has enabled Forward OAuth Identity (oauthPassThru).
Clear auth headers: Will clear any outgoing HTTP headers that was part of the incoming
HTTP request and used when authenticating to Grafana.
The current suggested way to register client middlewares is to have a separate package,
pluginsintegration, responsible for bootstrap/instantiate the backend plugin client with
middlewares and/or longer term bootstrap/instantiate plugin management.
Fixes#54135
Related to #47734
Related to #57870
Related to #41623
Related to #57065
