Add buildkite functionality to persist and restore lagrange basis cache

[martyall]

We are storing the results of the lagrange basis computations when running dev unit tests (see #15802). Currently the tests lazily populate the cache, so that the first time you need the basis you compute and every other time you load from disk. We lose the disk at the end of the run,.

We would like to investigate persisting and restoring this cache for the dev unit tests since these basically never change. Ideally the flow would be something like

• restore the cache to proper directory via s3, nfs or whatever is easy
• run tests
  [ IDEALLY ]
• if the cache has changed (this can happen when new tests are added), push to store in smart way

NOTE: The total cache size is currently 8kb and not expected to grow over time.

[martyall]

depends on #15843