Allow spark expressions in field mapping during Ingestion

[pyalex]

Signed-off-by: Oleksii Moskalenko moskalenko.alexey@gmail.com

What this PR does / why we need it:

Instead of just map columns with different name - this PR allows to use expressions (spark style) to enable simple transformations of features during ingestion.
Example

fieldMapping = {
   "feature1": "cos(feature1)",
   "feature2": "feature2 / feature1"
}

Available functions: https://spark.apache.org/docs/2.3.1/api/sql/index.html

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

[khorshuheng]

/test python-sdk-integration-test

[woop]

/lgtm

[pyalex]

/test python-sdk-integration-test

[feast-ci-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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