sysroot: Add concept of deployment "pinning" 📌

Example user story: Jane rebases her OS to a new major version N, and wants to
keep around N-1 even after a few upgrades for a while so she can easily roll
back. I plan to add `rpm-ostree rebase --pin` to opt-in to this for example.

Builds on the new `libostree-transient` group to store pinning state there.

Closes: https://github.com/ostreedev/ostree/issues/1460

Closes: #1464
Approved by: jlebon

apidoc/ostree-sections.txt

@@ -169,6 +169,7 @@ ostree_deployment_get_bootconfig
 ostree_deployment_get_origin
 ostree_deployment_get_origin_relpath
 ostree_deployment_get_unlocked
+ostree_deployment_is_pinned
 ostree_deployment_set_index
 ostree_deployment_set_bootserial
 ostree_deployment_set_bootconfig
@@ -509,6 +510,7 @@ ostree_sysroot_init_osname
 ostree_sysroot_deployment_set_kargs
 ostree_sysroot_deployment_set_mutable
 ostree_sysroot_deployment_unlock
+ostree_sysroot_deployment_set_pinned
 ostree_sysroot_write_deployments
 ostree_sysroot_write_deployments_with_options
 ostree_sysroot_write_origin_file