use correct :scheme header in async_client requests

*Title*: *use correct :scheme header in async_client requests*

*Description*:
This is a follow up of PR #37978

There I discovered, that requests of the jwks_fetcher did not set the proper :scheme pseudo header, according to the configured URI (it was always 'http' even the transport was 'https'). This lead to problems if http/2 was higher is being used with some servers. For the jwks_fetcher this was fixed in #37978.

But this is a general issue with other 'envoy originating' requests. For example the token endpoint requests of the oauth2 filter will will have the same problem.

In the PR I extended the `Http::Utility::prepareHeaders()` function to also generate the :scheme header according to the given URI (if the optional `include_scheme` flag is set).

So to find and fix other affected areas one could search the code for usages of the `prepareHeaders()` function and consider if this is an affected request and set the `include_scheme` flag.

Since I will not be able to do it myself in the foreseeable future I open this Issue so maybe some else might do it or at least there is an issue to track this.

Regards