Feature Request: Support $HOME/.kube/config.d/*

Greetings!

kubectl already allows defining multiple clusters and users in `$HOME/.kube/config` however editing this file by hand or even by tools is a bit cumbersome.

If `kubectl` supported loading multiple config files from `$HOME/.kube/config.d/` it would make dealing with different cluster configurations much easier.

For example,

1. kubespray already generates a config file, but it is still not easy to use it if you already have a config file setup (kubernetes-sigs/kubespray/pull/1647).

2. aws eks cli already mutates the config file, but dealing with multiple clusters or updating this information requires way more mental overhead than it should require.

I would like to hear some feedback on this and whatever a pull request would be considered.

Many Thanks!