feat: Add default and custom UKI Spash Screen Image Support

[bencorrado]

Is your feature request related to a problem? Please describe.
This is more an enhancement than a problem. Currently when booting a UKI image, during the image loading and verification steps it shows a slash screen with red "UKIFY" branding. This is confusing to users who don't know what a UKI is and the use of red lettering usually indicates an error, so users assume something is wrong during the long boot.

Describe the solution you'd like
It would be great to default to using a BMP with "KAIROS" branding and some indication of "Booting..."

Currently enki does not support injecting a BMP file for this, so this functionality will need to be added.

It would also be good to add a flag to allow building with a custom BMP file similar to the --ENKI_FLAGS="--boot-branding \"Custom OS\"" where a user could link in their own BMP.

Describe alternatives you've considered
If there is not a desire to modify enki he Earthfile can be modified for development users to use ukify directly to do this in the uki-dev-build stage:

    # Create the directory for the BMP file
    RUN mkdir -p /usr/share/systemd/bootctl

    # Copy the BMP file into the directory
    COPY ../custom_splash.bmp /usr/share/systemd/bootctl/custom_splash.bmp

    COPY +git-version/GIT_VERSION ./
    ARG KAIROS_VERSION=$(cat GIT_VERSION)

    ARG UNAME=$(cat Uname)
    RUN /usr/lib/systemd/ukify Kernel initrd \
        --cmdline=@Cmdline \
        --os-release=@Osrelease \
        --uname="${UNAME}" \
        --stub /usr/lib/systemd/boot/efi/linuxx64.efi.stub \
        --secureboot-private-key DB.key \
        --secureboot-certificate DB.crt \
        --pcr-private-key tpm2-pcr-private.pem \
        --measure \
        --splash=/usr/share/systemd/bootctl/custom_splash.bmp \
        --output uki.signed.efi

Additional context