Add `get_samplesheet_constructor` for Nextflow-on-Latch workflows

[ameynert]

Get the samplesheet constructor from the Nextflow metadata.

`metadata._nextflow_metadata` is a global variable within the Latch environment that will be
populated for a Nextflow-on-Latch execution. Within this metadata we have access to the
parameters declared in `latch_metadata/parameter.py`.

When supporting import from Latch Registry as an input to the Nextflow-on-Latch workflow,
records from the Registry must be serialized to file so they may be passed to Nextflow as a
samplesheet. All `samplesheet` `NextflowParameters` have an associated constructor that maps a
list of the corresponding dataclasses to a serialized file.

This function verifies the existence of an appropriate constructor on the Nextflow metadata and
returns it.

Notes:

• will have to pass metadata to this function rather than accessing it as a global?