[SEDONA-230] rdd.saveAsGeoJSON should generate feature properties with field names

[jiayuasu]

Did you read the Contributor Guide?

• Yes, I have read Contributor Rules and Contributor Development Guide

Is this PR related to a JIRA ticket?

• Yes, the URL of the assoicated JIRA ticket is https://issues.apache.org/jira/browse/SEDONA-230. The PR name follows the format [SEDONA-230] my subject.

What changes were proposed in this PR?

1. SpatialRdd that has fields will be saved as a geojson that has correct property keys and values for each record.
2. Null field value will be saved as empty space. E.g.,

"properties": {"name": ""}

3. SpatialRdd that has no fields will be save as

"properties": null

One remaining issue is that numerical values in geojson will be converted to string type. To solve this, we need the full schema info preserved in SpatialRDD metadata. I will leave in this as the future work.

How was this patch tested?

Add a few more tests and passed existing tests

Did this PR include necessary documentation updates?

• No, this PR does not affect any public API so no need to change the docs.