Add support for extraFiles for the binderhub pod

[consideRatio]

The hub.extraFiles and singleuser.extraFiles from z2jh's chart has been a success, we should allow the binderhub pod to use the same by adding extraFiles config I think.

This can be done by:

• copy-pasting and renaming these chart helper functions from z2jh
  This is already done because we already use these helpers for dockerApi.extraFiles.
• declaring an empty dict entry in values.yaml, values.schema.yaml, and tools/templates/lint-and-validate-values.yaml for extraFiles, reusing whats set for dockerApi.extraFiles
• using them like done in the associated pod's k8s Secret
• mounting them like done in the associated pod