kernel-install: Add --json option for inspect verb

kernel-install: Add --json option for inspect verb

In mkosi, we can't use kernel-install directly but we do want to
mimick its behavior as much as possible. Let's make that easier by
making it easy to fetch data from kernel-install as JSON.

To get all the necessary data, we extend the inspect verb to also
allow passing in a kernel version and initrds, to mimick the "add"
verb. The kernel version is used to determine the "Entry Directory",
and in absence of auto-detection of kernel version in kernel-install
we have to allow users to pass it.

We also add --no-pager while we're at it.