interfaces/builtin: Add `tmpfs-mount` interface

[fnordahl]

Multiple software projects make use of tmpfs as part of their operation. Some notable examples are Ceph and Kubernetes.

We already have mount/umount of tmpfs support added as part of the kubernetes-support interface and with this PR I propose we add a separate interface for it so that multiple snaps can consume it.

In reality it is a copy of the cifs-mount interface so it may call for creating a more generic interface where fstype is a dynamic property of the interface, let's discuss on the PR.