[10.x] Add PendingRequest withHeader() method

[ralphjsmit]

This PR adds a convenience withHeader() method on the Http client PendingRequest class. I think that the majority of the time people only add a single header when using the withHeaders() method, so we could as well have a convenient method that saves 2 lines.

Before:

Http::baseUrl(config('services.active-campaign.endpoint') . '/api/3')
    ->withHeaders([
        'Api-Token' => config('services.active-campaign.token')
    ])
    ->acceptJson();

After:

Http::baseUrl(config('services.active-campaign.endpoint') . '/api/3')
    ->withHeader('Api-Token', config('services.active-campaign.token'))
    ->acceptJson();

Thanks!