By specifying a function typed `LookupFunction` the caller can customize
which path expressions to interpolate and which ones to skip. When we
express dependencies between resources their values are known by
Terraform at deploy time. Therefore, we have to skip interpolation for
`${resources.jobs.my_job.id}` and instead rewrite it to
`${databricks_job.my_job.id}` before passing it along to Terraform.
Performs interpolation on string field.
It looks for patterns `${foo.bar}` where `foo.bar` points to a string
field in the configuration data model.
It does not support traversal (e.g. `${foo}` with `foo` equal
to`${bar}`), hence "rudimentary".
Pieter Noordhuis