Custom transforms - adding a sample for a simple custom visual transform

[rmattsampson]

Issue #, if available:

Description of changes: Adding a basic sample custom visual transform for Glue Studio. Particular sample fills empty strings in a user defined column with a user defined value Custom Visual Transforms general guidelines and creation docs are available here: https://docs.aws.amazon.com/glue/latest/ug/custom-visual-transform-create-gs.html

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[rmattsampson]

Received some feedback offline, will apply that to this PR and get a new iteration

[rmattsampson]

I have re-tested this code (using the CVT process to add a CVT, and have it tested on some sample data set)

I have simplified the code quite a bit

The CVT now handles "nulls" or "empty strings" the same. There is a dependency on PANDAS now (for the null case)

[rmattsampson]

I have REMOVED the PANDAS Dependency on fillna, and instead use the PySpark FillNa method which works just as well tested using Jupyter Notebooks and of course CVT directly in Glue Studio

[moomindani]

Thank you for your contribution! Merged this PR.