Add support for an ~/.aws/config.d/ directory

**Is your feature request related to a problem? Please describe.**

My ~/.aws/config file is massive. I have a proliferation of personal accounts, work accounts, and each client grants me a profile within dozens of accounts as well. I'd like to bring some order to that chaos, as well as not have a single file that lists every AWS account to which I currently have configured access.

**Describe the solution you'd like**
Being able to break out profiles on a per-client, or along a personal / work divide via different configuration files enables a number of useful workflows. I'd like a subdirectory within ~/.aws/ named `config.d`; files within that directory can then be included when ~/.aws/config is read by various ecosystem tools. They can be independently managed, and not result in the obnoxious and manual / error prone approach of either editing the file by hand to reflect new accounts / the removal of old ones, or the somehow-worse problem of building a variety of "aws config file generator" scripts bespoke to every customer who writes one of the blasted things.

**Describe alternatives you've considered**
I have tried and discarded:
* Generating the config file from scripts
* Ignoring the problem in the forlorn hope it would annoy me less with time
* manually editing the file
* hiring an assistant to log me into AWS accounts manually so I don't have to worry about it

**Additional context**
The `conf.d/` structure is [well understood](https://www.redhat.com/sysadmin/etc-configuration-directories) within Linux, and is a well adopted pattern ecosystem-wide.