redhat/kernel.spec: add uki_addons to create UKI kernel cmdline addons

JIRA: https://issues.redhat.com/browse/RHEL-45159 
JIRA: https://issues.redhat.com/browse/RHEL-45160

We want to enable kernel.spec to optionally ship UKI addons defined in a common config file in redhat folder.
The folder redhat/uki_addons will contain all addons configs specifying the
UKI kernel cmdline addons to be created in the next build. 

An addon config is simply a .addon plain text file, where any line
is taken as kernel cmdline, except for the ones starting with '#',
which will be automatically ignored.

redhat/scripts/uki_create_json.py will take care of parsing all configs and
folders in redhat/uki_addons and produce a json file 'uki_addons.json'
that is then added into the SRPM.

When building the RPM, kernel.spec will then call uki_create_addons.py
to parse the json and call 'ukify' for each addon matching the given
distro, arch and uki provided in input.

The output addon filename will be a concatenation of all folders in
redhat/uki_addons that are part of the addon config path.
The folder hierarchy inside of redhat/uki_addons is similar to
redhat/configs: distro/distro/distro/UKI_NAME/%arch.

It is also possible to add .sbat to all the generated addons, by
populating redhat/uki_addons/distro/distro/distro/UKI_NAME/%arch/sbat/sbat.conf.
Syntax is same as the addons config.

As an example of this feature, add the fips addon to optionally enable fips (https://issues.redhat.com/browse/RHEL-45160).

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>