[Feature Request] Add webhook support

[antheas]

While COPR based kernels and fedora kernels do not support webhooks and can be run manually, kernels built on github do.

Moreover, we might want to build testing kernels that should not be pushed to users under the latest tag. Therefore, add a webhook that receives a kernel flavor and a build tag, and commits it to the kernel cache.

Afterwards, it should call the webhook for akmods (see ublue-os/akmods#256), so that akmods for that kernel can build automatically.

Overall, this should reduce the kernel iteration time from an average of 5 days (as it is right now with fsync) to 2 and a half hours (1:50 for kernel build, 10m for kernel-cache, and 10m for akmods).

Needs ublue-os/akmods#256