Add an API+CLI to inject metadata for bootable OSTree commits

I was doing some rpm-ostree work and I wanted to compare two
OSTree commits to see if the kernel has changed.  I think
this should be a lot more natural.

Add `ostree commit --bootable` which calls into a new generic
library API `ostree_commit_metadata_for_bootable()` that
discovers the kernel version and injects it as an `ostree.linux`
metadata key.  And for extra clarity, add an `ostree.bootable`
key.

It's interesting because the "core" OSTree layer is all about
generic files, but this is adding special APIs around bootable
OSTree commits (as opposed to e.g. flatpak as well as
things like rpm-ostree's pkgcache refs).

Eventually, I'd like to ensure everyone is using this and
hard require this metadata key for the `ostree admin deploy`
flow - mainly to prevent accidents.

apidoc/ostree-sections.txt

@@ -152,6 +152,7 @@ ostree_validate_structureof_dirtree
 ostree_validate_structureof_dirmeta
 ostree_commit_get_parent
 ostree_commit_get_timestamp
+ostree_commit_metadata_for_bootable
 ostree_commit_get_content_checksum
 ostree_commit_get_object_sizes
 OstreeCommitSizesEntry