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

src/libostree/ostree-sysroot.h

@@ -181,6 +181,12 @@ gboolean ostree_sysroot_deployment_set_mutable (OstreeSysroot     *self,
                                                 GCancellable      *cancellable,
                                                 GError           **error);
 
+_OSTREE_PUBLIC
+gboolean ostree_sysroot_deployment_set_pinned (OstreeSysroot     *self,
+                                               OstreeDeployment  *deployment,
+                                               gboolean           is_pinned,
+                                               GError           **error);
+
 _OSTREE_PUBLIC
 gboolean ostree_sysroot_deployment_unlock (OstreeSysroot     *self,
                                            OstreeDeployment  *deployment,