[RFE] The composer needs a command to clear the separate cache

Description of problem:
Customer was not able to generate image with the latest kernel whereas a more recent version was available in the repositories.
Erasing the cache and then restarting the daemon fixed the issue.
A command allowing to erase it from the `composer-cli` would be welcome to ease that part, and that would let the users know a separate cache is maintained by the composer.

Version-Release number of selected component (if applicable):
composer-cli-28.14.62-1.el8.x86_64
osbuild-composer-33.2-1.el8.x86_64

How reproducible:
Always

Steps to Reproduce:
Use a VM with with old metadata in the composer separate cache and run a depsolve.

Actual results:

1. yum list kernel
   Updating Subscription Management repositories.
   Last metadata expiration check: 2:47:07 ago on Tue 03 May 2022 07:43:52 AM CEST.
   Installed Packages
   kernel.x86_64 4.18.0-305.el8
   kernel.x86_64 4.18.0-348.12.2.el8_5
   Available Packages
   kernel.x86_64 4.18.0-348.23.1.el8_5 <===

1. composer-cli blueprints depsolve working | grep kernel
   kernel-modules-4.18.0-348.20.1.el8_5.x86_64
   kernel-core-4.18.0-348.20.1.el8_5.x86_64
   kernel-4.18.0-348.20.1.el8_5.x86_64 <===

Expected results:
The latest kernel is picked up.

Additional info:

• To workaround the issue:

1. rm -rf /var/cache/osbuild-composer/*
2. systemctl restart osbuild-composer

• The metadata_expire DNF option is maybe too briefly mentioned in the doc.