sysroot: Add concept of deployment "pinning" 📌

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